Method and apparatus for transmitting and receiving frame including partial association identifier in wireless lan system

ABSTRACT

A method and apparatus for transmitting and receiving a frame including a partial association ID (PAID) in a wireless LAN (WLAN) system are disclosed. A method for receiving a frame at a station (STA) from an access point (AP) of a wireless communication system including: determining whether a value of a partial association ID (Partial AID) of the frame is calculated on the basis of an association ID (AID) allocated to the STA by the AP and a basic service set ID (BSSID) of the AP; and decoding the frame, if the value of the partial AID of the frame is calculated on the basis of the AID allocated to the STA and the BSSID of the AP, wherein the AID is allocated such that the partial AID value calculated on the basis of the AID allocated to the STA and the BSSID of the AP is not identical to a first PAID value calculated by applying a modulo operation to a specific value obtained by converting values ranging from a 40 th  bit position to a 48 th  bit position from among 48 bit positions of the BSSID of the AP into a decimal number.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly to a method and apparatus for transmitting andreceiving a frame including a partial association identifier (PAID) in awireless LAN (WLAN) system.

BACKGROUND ART

Various wireless communication technologies systems have been developedwith rapid development of information communication technologies. WLANtechnology from among wireless communication technologies allowswireless Internet access at home or in enterprises or at a specificservice provision region using mobile terminals, such as a PersonalDigital Assistant (PDA), a laptop computer, a Portable Multimedia Player(PMP), etc, on the basis of Radio Frequency (RF) technology.

In order to obviate limited communication speed, one of the advantagesof WLAN, the recent technical standard has proposed an evolved systemcapable of increasing the speed and reliability of a network whilesimultaneously extending a coverage region of a wireless network. Forexample, IEEE 802.11n enables a data processing speed to support amaximum high throughput (HT) of 540 Mbps. In addition, Multiple Inputand Multiple Output (MIMO) technology has recently been applied to botha transmitter and a receiver so as to minimize transmission errors aswell as to optimize a data transfer rate.

DISCLOSURE Technical Problem

Accordingly, the present invention is directed to a method and apparatusfor transmitting and receiving a frame including a partial associationidentifier (PAID) in a WLAN system that substantially obviate one ormore problems due to limitations and disadvantages of the related art.Machine to Machine (M2M) communication technology has been discussed asnext generation communication technology. A technical standard forsupporting M2M communication in IEEE 802.11 WLAN has been developed asIEEE 802.11ah. M2M communication may sometimes consider a scenariocapable of communicating a small amount of data at low speed in anenvironment including a large number of devices.

WLAN communication is carried out in mediums shared among all devices.In case of increasing the number of devices as in M2M communication,much time is taken to access a channel of one device, such that overallsystem performance is unavoidably deteriorated, resulting in difficultyin power saving of each device.

An object of the present invention is to provide a method forconfiguring a frame including a partial association identifier (PAID).

It is to be understood that technical objects to be achieved by thepresent invention are not limited to the aforementioned technicalobjects and other technical objects which are not mentioned herein willbe apparent from the following description to one of ordinary skill inthe art to which the present invention pertains.

Technical Solution

The object of the present invention can be achieved by providing amethod for receiving a frame at a station (STA) from an access point(AP) of a wireless communication system including: determining whether avalue of a partial association ID (Partial AID) of the frame iscalculated on the basis of an association ID (AID) allocated to the STAby the AP and a basic service set ID (BSSID) of the AP; and decoding theframe, if the value of the partial AID of the frame is calculated on thebasis of the AID allocated to the STA and the BSSID of the AP, whereinthe AID is allocated such that the partial AID value calculated on thebasis of the AID allocated to the STA and the BSSID of the AP is notidentical to a first PAID value calculated by applying a modulooperation to a specific value obtained by converting values ranging froma 40^(th) bit position to a 48^(th) bit position from among 48 bitpositions of the BSSID of the AP into a decimal number.

In another aspect of the present invention, a method for transmitting aframe to a station (STA) at an access point (AP) of a wirelesscommunication system includes: calculating, by the access point (AP), apartial association ID (Partial AID) on the basis of an association ID(AID) allocated to the STA and a basic service set ID (BSSID) of the AP;and transmitting the frame including a partial AID field set to aspecific value corresponding to the calculation result of the partialAID to the STA, wherein the AID is allocated such that the partial AIDvalue calculated on the basis of the AID allocated to the STA and theBSSID of the AP is not identical to a first PAID value calculated byapplying a modulo operation to a specific value obtained by convertingvalues ranging from a 40^(th) bit position to a 48^(th) bit positionfrom among 48 bit positions of the BSSID of the AP into a decimalnumber.

In another aspect of the present invention, a station (STA) device forreceiving a frame from an access point (AP) of a wireless communicationsystem includes: transceiver; and a processor, wherein the processordetermines whether a value of a partial association ID (Partial AID) ofthe frame is calculated on the basis of an association ID (AID)allocated to the STA by the AP and a basic service set ID (BSSID) of theAP; and decodes the frame, if the value of the partial AID of the frameis calculated on the basis of the AID allocated to the STA and the BSSIDof the AP, wherein the AID is allocated such that the partial AID valuecalculated on the basis of the AID allocated to the STA and the BSSID ofthe AP is not identical to a first PAID value calculated by applying amodulo operation to a specific value obtained by converting valuesranging from a 40^(th) bit position to a 48^(th) bit position from among48 bit positions of the BSSID of the AP into a decimal number.

In another aspect of the present invention, an access point (AP) devicefor transmitting a frame to a station (STA) of a wireless communicationsystem includes: a transceiver; and a processor, wherein the processorcalculates a partial association ID (Partial AID) on the basis of anassociation ID (AID) allocated to the STA by the AP and a basic serviceset ID (BSSID) of the AP, and transmits the frame including a partialAID field set to a specific value corresponding to the calculationresult of the partial AID to the STA, wherein the AID is allocated suchthat the partial AID value calculated on the basis of the AID allocatedto the STA and the BSSID of the AP is not identical to a first PAIDvalue calculated by applying a modulo operation to a specific valueobtained by converting values ranging from a 40^(th) bit position to a48^(th) bit position from among 48 bit positions of the BSSID of the APinto a decimal number.

The following description may be commonly applied to the embodiments ofthe present invention.

The AID may be allocated such that the partial AID value is notidentical to a second PAID value calculated by applying a modulooperation to a specific value obtained by converting values ranging froma 40^(th) bit position to a 48^(th) bit position from among 48 bitpositions of a BSSID of an Overlapping BSS (OBSS) into a decimal number.

The first PAID value may be calculated by(dec(BSSID[39:47])mod(2⁹-1))+1, where BSSID is the BSSID of the AP,dec(A) is a specific value obtained by converting A into a decimalnumber, A[b:c] is bits from Bit B to Bit C of A when a first bit of abinary number A is denoted by Bit 0, and ‘mod.’ denotes a modulooperation.

The second PAID value may be calculated by (dec(OBSSBSSID[39:47])mod(2⁹−1))+1, where OBSS BSSID is the BSSID of the OBSS,dec(A) is a specific value obtained by converting A into a decimalnumber, A[b:c] is bits from Bit B to Bit C of A when a first bit of abinary number A is denoted by Bit 0, and ‘mod’ denotes a modulooperation.

The partial AID value calculated on the basis of the AID allocated tothe STA and the BSSID of the AP may be calculated bydec(AID[0:8]+dec(BSSID[44:47] XOR BSSID[40:43])×2⁵)mod 2⁹, where AID isthe AID allocated to the STA, BSSID is the BSSID of the AP, dec(A) is aspecific value obtained by converting A into a decimal number, A[b:c] isbits from Bit B to Bit C of A when a first bit of a binary number A isdenoted by Bit 0, and ‘mod’ denotes a modulo operation.

The AP may be an AP associated with the STA.

The AID may be allocated such that the partial AID calculated on thebasis of the AID allocated to the STA and the BSSID of the AP is notidentical to zero (0).

The partial AID field may be included in a Signal A (SIG-A) field of theframe.

The frame may be a single user (SU) frame.

The frame may be defined at a sub-1 GHz operation frequency.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

Advantageous Effects

As is apparent from the above description, exemplary embodiments of thepresent invention may provide a method and apparatus for constructing aframe including a partial association ID (PAID).

It will be appreciated by persons skilled in the art that the effectsthat can be achieved with the present invention are not limited to whathas been particularly described hereinabove and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

FIG. 1 exemplarily shows an IEEE 802.11 system according to oneembodiment of the present invention.

FIG. 2 exemplarily shows an IEEE 802.11 system according to anotherembodiment of the present invention.

FIG. 3 exemplarily shows an IEEE 802.11 system according to stillanother embodiment of the present invention.

FIG. 4 is a conceptual diagram illustrating a WLAN system.

FIG. 5 is a flowchart illustrating a link setup process for use in theWLAN system.

FIG. 6 is a conceptual diagram illustrating a backoff process.

FIG. 7 is a conceptual diagram illustrating a hidden node and an exposednode.

FIG. 8 is a conceptual diagram illustrating RTS (Request To Send) andCTS (Clear To Send).

FIG. 9 is a conceptual diagram illustrating a power managementoperation.

FIGS. 10 to 12 are conceptual diagrams illustrating detailed operationsof a station (STA) having received a Traffic Indication Map (TIM).

FIG. 13 is a conceptual diagram illustrating a group-based AID.

FIG. 14 exemplarily shows SU/MU frame formats.

FIG. 15(a) exemplarily shows an example of an AID reassignment requestframe format, and FIG. 15(b) exemplarily shows an example of an AIDreassignment response frame format.

FIG. 16 exemplarily shows fixed sequences available as an SIG-B field.

FIG. 17 is a conceptual diagram illustrating a method repeated in anSIG-B field when a fixed pattern of FIG. 16 is transferred to PPDU.

FIG. 18 is a flowchart illustrating a method for transmitting andreceiving a frame according to one embodiment of the present invention.

FIG. 19 is a block diagram illustrating a radio frequency (RF) deviceaccording to one embodiment of the present invention.

BEST MODE

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. The detailed description, which will be given below withreference to the accompanying drawings, is intended to explain exemplaryembodiments of the present invention, rather than to show the onlyembodiments that can be implemented according to the present invention.The following detailed description includes specific details in order toprovide a thorough understanding of the present invention. However, itwill be apparent to those skilled in the art that the present inventionmay be practiced without such specific details.

The following embodiments are proposed by combining constituentcomponents and characteristics of the present invention according to apredetermined format. The individual constituent components orcharacteristics should be considered optional factors on the conditionthat there is no additional remark. If required, the individualconstituent components or characteristics may not be combined with othercomponents or characteristics. In addition, some constituent componentsand/or characteristics may be combined to implement the embodiments ofthe present invention. The order of operations to be disclosed in theembodiments of the present invention may be changed. Some components orcharacteristics of any embodiment may also be included in otherembodiments, or may be replaced with those of the other embodiments asnecessary.

It should be noted that specific terms disclosed in the presentinvention are proposed for convenience of description and betterunderstanding of the present invention, and the use of these specificterms may be changed to other formats within the technical scope orspirit of the present invention.

In some instances, well-known structures and devices are omitted inorder to avoid obscuring the concepts of the present invention andimportant functions of the structures and devices are shown in blockdiagram form. The same reference numbers will be used throughout thedrawings to refer to the same or like parts.

Exemplary embodiments of the present invention are supported by standarddocuments disclosed for at least one of wireless access systemsincluding an Institute of Electrical and Electronics Engineers (IEEE)802 system, a 3^(rd) Generation Partnership Project (3GPP) system, a3GPP Long Term Evolution (LTE) system, an LTE-Advanced (LTE-A) system,and a 3GPP2 system. In particular, steps or parts, which are notdescribed to clearly reveal the technical idea of the present invention,in the embodiments of the present invention may be supported by theabove documents. All terminology used herein may be supported by atleast one of the above-mentioned documents.

The following embodiments of the present invention can be applied to avariety of wireless access technologies, for example, CDMA (CodeDivision Multiple Access), FDMA (Frequency Division Multiple Access),TDMA (Time Division Multiple Access). OFDMA (Orthogonal FrequencyDivision Multiple Access), SC-FDMA (Single Carrier Frequency DivisionMultiple Access), and the like. CDMA may be embodied through wireless(or radio) technology such as UTRA (Universal Terrestrial Radio Access)or CDMA2000. TDMA may be embodied through wireless (or radio) technologysuch as GSM (Global System for Mobile communication)/GPRS (GeneralPacket Radio Service)/EDGE (Enhanced Data Rates for GSM Evolution).OFDMA may be embodied through wireless (or radio) technology such asInstitute of Electrical and Electronics Engineers (IEEE) 802.11 (Wi-Fi),IEEE 802.16 (WiMAX), IEEE 802-20, and E-UTRA (Evolved UTRA). Forclarity, the following description focuses on IEEE 802.11 systems.However, technical features of the present invention are not limitedthereto.

WLAN System Structure

FIG. 1 exemplarily shows an IEEE 802.11 system according to oneembodiment of the present invention.

The structure of the IEEE 802.11 system may include a plurality ofcomponents. A WLAN which supports transparent STA mobility for a higherlayer may be provided by mutual operations of the components. A BasicService Set (BSS) may correspond to a basic constituent block in an IEEE802.11 LAN. In FIG. 1, two BSSs (BSS1 and BSS2) are shown and two STAsare included in each of the BSSs (i.e. STA1 and STA2 are included inBSS1 and STA3 and STA4 are included in BSS2). An ellipse indicating theBSS in FIG. 1 may be understood as a coverage area in which STAsincluded in the corresponding BSS maintain communication. This area maybe referred to as a Basic Service Area (BSA). If an STA moves out of theBSA, the STA cannot directly communicate with the other STAs in thecorresponding BSA.

In the IEEE 802.11 LAN, the most basic type of BSS is an Independent BSS(IBSS). For example, the IBSS may have a minimum form consisting of onlytwo STAs. The BSS (BSS1 or BSS2) of FIG. 1, which is the simplest formand in which other components are omitted, may correspond to a typicalexample of the IBSS. Such configuration is possible when STAs candirectly communicate with each other. Such a type of LAN is notprescheduled and may be configured when the LAN is necessary. This maybe referred to as an ad-hoc network.

Memberships of an STA in the BSS may be dynamically changed when the STAis switched on or off or the STA enters or leaves the BSS region. TheSTA may use a synchronization process to join the BSS. To access allservices of a BSS infrastructure, the STA should be associated with theBSS. Such association may be dynamically configured and may include useof a Distribution System Service (DSS).

FIG. 2 is a diagram showing another exemplary structure of an IEEE802.11 system to which the present invention is applicable. In FIG. 2,components such as a Distribution System (DS), a Distribution SystemMedium (DSM), and an Access Point (AP) are added to the structure ofFIG. 1.

A direct STA-to-STA distance in a LAN may be restricted by PHYperformance. In some cases, such restriction of the distance may besufficient for communication. However, in other cases, communicationbetween STAs over a long distance may be necessary. The DS may beconfigured to support extended coverage.

The DS refers to a structure in which BSSs are connected to each other.Specifically, a BSS may be configured as a component of an extended formof a network consisting of a plurality of BSSs, instead of independentconfiguration as shown in FIG. 1.

The DS is a logical concept and may be specified by the characteristicof the DSM. In relation to this, a Wireless Medium (WM) and the DSM arelogically distinguished in IEEE 802.11. Respective logical media areused for different purposes and are used by different components. Indefinition of IEEE 802.11, such media are not restricted to the same ordifferent media. The flexibility of the IEEE 802.11 LAN architecture (DSarchitecture or other network architectures) can be explained in that aplurality of media is logically different. That is, the IEEE 802.11 LANarchitecture can be variously implemented and may be independentlyspecified by a physical characteristic of each implementation.

The DS may support mobile devices by providing seamless integration ofmultiple BSSs and providing logical services necessary for handling anaddress to a destination.

The AP refers to an entity that enables associated STAs to access the DSthrough a WM and that has STA functionality. Data may move between theBSS and the DS through the AP. For example, STA2 and STA3 shown in FIG.2 have STA functionality and provide a function of causing associatedSTAs (STA1 and STA4) to access the DS. Moreover, since all APscorrespond basically to STAs, all APs are addressable entities. Anaddress used by an AP for communication on the WM need not always beidentical to an address used by the AP for communication on the DSM.

Data transmitted from one of STAs associated with the AP to an STAaddress of the AP may always be received by an uncontrolled port and maybe processed by an IEEE 802.1X port access entity. If the controlledport is authenticated, transmission data (or frame) may be transmittedto the DS.

FIG. 3 is a diagram showing still another exemplary structure of an IEEE802.11 system to which the present invention is applicable. In additionto the structure of FIG. 2, FIG. 3 conceptually shows an ExtendedService Set (ESS) for providing wide coverage.

A wireless network having arbitrary size and complexity may be comprisedof a DS and BSSs. In the IEEE 802.11 system, such a type of network isreferred to an ESS network. The ESS may correspond to a set of BSSsconnected to one DS. However, the ESS does not include the DS. The ESSnetwork is characterized in that the ESS network appears as an IBSSnetwork in a Logical Link Control (LLC) layer. STAs included in the ESSmay communicate with each other and mobile STAs are movabletransparently in LLC from one BSS to another BSS (within the same ESS).

In IEEE 802.11, relative physical locations of the BSSs in FIG. 3 arenot assumed and the following forms are all possible. BSSs may partiallyoverlap and this form is generally used to provide continuous coverage.BSSs may not be physically connected and the logical distances betweenBSSs have no limit. BSSs may be located at the same physical positionand this form may be used to provide redundancy. One or more IBSSs orESS networks may be physically located in the same space as one or moreESS networks. This may correspond to an ESS network form in the case inwhich an ad-hoc network operates in a location in which an ESS networkis present, the case in which IEEE 802.11 networks of differentorganizations physically overlap, or the case in which two or moredifferent access and security policies are necessary in the samelocation.

FIG. 4 is a diagram showing an exemplary structure of a WLAN system. InFIG. 4, an example of an infrastructure BSS including a DS is shown.

In the example of FIG. 4, BSS1 and BSS2 constitute an ESS. In the WLANsystem, an STA is a device operating according to MAC/PHY regulation ofIEEE 802.11. STAs include AP STAs and non-AP STAs. The non-AP STAscorrespond to devices, such as laptop computers or mobile phones,handled directly by users. In FIG. 4, STA1, STA3, and STA4 correspond tothe non-AP STAs and STA2 and STA5 correspond to AP STAs.

In the following description, the non-AP STA may be referred to as aterminal, a Wireless Transmit/Receive Unit (WTRU), a User Equipment(UE), a Mobile Station (MS), a mobile terminal, or a Mobile SubscriberStation (MSS). The AP is a concept corresponding to a Base Station (BS),a Node-B, an evolved Node-B (e-NB), a Base Transceiver System (BTS), ora femto BS in other wireless communication fields.

Link Setup Process

FIG. 5 is a flowchart explaining a general link setup process accordingto an exemplary embodiment of the present invention.

In order to allow an STA to establish link setup on the network as wellas to transmit/receive data over the network, the STA must perform suchlink setup through processes of network discovery, authentication, andassociation, and must establish association and perform securityauthentication. The link setup process may also be referred to as asession initiation process or a session setup process. In addition, anassociation step is a generic term for discovery, authentication,association, and security setup steps of the link setup process.

Link setup process is described referring to FIG. 5.

In step S510, STA may perform the network discovery action. The networkdiscovery action may include the STA scanning action. That is, STA mustsearch for an available network so as to access the network. The STAmust identify a compatible network before participating in a wirelessnetwork. Here, the process for identifying the network contained in aspecific region is referred to as a scanning process.

The scanning scheme is classified into active scanning and passivescanning.

FIG. 5 is a flowchart illustrating a network discovery action includingan active scanning process. In the case of the active scanning, an STAconfigured to perform scanning transmits a probe request frame and waitsfor a response to the probe request frame, such that the STA can movebetween channels and at the same time can determine which AP (AccessPoint) is present in a peripheral region. A responder transmits a proberesponse frame, acting as a response to the probe request frame, to theSTA having transmitted the probe request frame. In this case, theresponder may be an STA that has finally transmitted a beacon frame in aBSS of the scanned channel. In BSS, since the AP transmits the beaconframe, the AP operates as a responder. In IBSS, since STAs of the IBSSsequentially transmit the beacon frame, the responder is not constant.For example, the STA, that has transmitted the probe request frame atChannel #1 and has received the probe response frame at Channel #1,stores BSS-associated information contained in the received proberesponse frame, and moves to the next channel (for example, Channel #2),such that the STA may perform scanning using the same method (i.e.,probe request/response transmission/reception at Channel #2).

Although not shown in FIG. 5, the scanning action may also be carriedout using passive scanning. An STA configured to perform scanning in thepassive scanning mode waits for a beacon frame while simultaneouslymoving from one channel to another channel. The beacon frame is one ofmanagement frames in IEEE 802.11, indicates the presence of a wirelessnetwork, enables the STA performing scanning to search for the wirelessnetwork, and is periodically transmitted in a manner that the STA canparticipate in the wireless network. In BSS, the AP is configured toperiodically transmit the beacon frame. In IBSS, STAs of the IBSS areconfigured to sequentially transmit the beacon frame. If each STA forscanning receives the beacon frame, the STA stores BSS informationcontained in the beacon frame, and moves to another channel and recordsbeacon frame information at each channel. The STA having received thebeacon frame stores BSS-associated information contained in the receivedbeacon frame, moves to the next channel, and thus performs scanningusing the same method.

In comparison between the active scanning and the passive scanning, theactive scanning is more advantageous than the passive scanning in termsof delay and power consumption.

After the STA discovers the network, the STA may perform theauthentication process in step S520. The authentication process may bereferred to as a first authentication process in such a manner that theauthentication process can be clearly distinguished from the securitysetup process of step S540.

The authentication process may include transmitting an authenticationrequest frame to an AP by the STA, and transmitting an authenticationresponse frame to the STA by the AP in response to the authenticationrequest frame. The authentication frame used for authenticationrequest/response may correspond to a management frame.

The authentication frame may include an authentication algorithm number,an authentication transaction sequence number, a state code, a challengetext, a Robust Security Network (RSN), a Finite Cyclic Group (FCG), etc.The above-mentioned information contained in the authentication framemay correspond to some parts of information capable of being containedin the authentication request/response frame, may be replaced with otherinformation, or may include additional information.

The STA may transmit the authentication request frame to the AP. The APmay decide whether to authenticate the corresponding STA on the basis ofinformation contained in the received authentication request frame. TheAP may provide the authentication result to the STA through theauthentication response frame.

After the STA has been successfully authenticated, the associationprocess may be carried out in step S530. The association process mayinvolve transmitting an association request frame to the AP by the STA,and transmitting an association response frame to the STA by the AP inresponse to the association request frame.

For example, the association request frame may include informationassociated with various capabilities, a beacon listen interval, aService Set Identifier (SSID), supported rates, supported channels, RSN,mobility domain, supported operating classes, a TIM (Traffic IndicationMap) broadcast request, interworking service capability, etc.

For example, the association response frame may include informationassociated with various capabilities, a state code, an Association ID(AID), supported rates, an Enhanced Distributed Channel Access (EDCA)parameter set, a Received Channel Power Indicator (RCPI), a ReceivedSignal to Noise Indicator (RSNI), mobility domain, a timeout interval(association comeback time), an overlapping BSS scan parameter, a TIMbroadcast response, a QoS map, etc.

The above-mentioned information may correspond to some parts ofinformation capable of being contained in the associationrequest/response frame, may be replaced with other information, or mayinclude additional information.

After the STA has been successfully associated with the network, asecurity setup process may be carried out in step S540. The securitysetup process of Step S540 may be referred to as an authenticationprocess based on Robust Security Network Association (RSNA)request/response. The authentication process of step S520 may bereferred to as a first authentication process, and the security setupprocess of Step S540 may also be simply referred to as an authenticationprocess.

For example, the security setup process of Step S540 may include aprivate key setup process through 4-way handshaking based on an(Extensible Authentication Protocol over LAN (EAPOL) frame. In addition,the security setup process may also be carried out according to othersecurity schemes not defined in IEEE 802.11 standards.

WLAN Evolution

In order to obviate limitations in WLAN communication speed, IEEE802.11n has recently been established as a communication standard. IEEE802.11n aims to increase network speed and reliability as well as toextend a coverage region of the wireless network. In more detail, IEEE802.11n supports a High Throughput (HT) of a maximum of 540 Mbps, and isbased on MIMO technology in which multiple antennas are mounted to eachof a transmitter and a receiver.

With the widespread use of WLAN technology and diversification of WLANapplications, there is a need to develop a new WLAN system capable ofsupporting a HT higher than a data processing speed supported by IEEE802.11n. The next generation WLAN system for supporting Very HighThroughput (VHT) is the next version (for example, IEEE 802.11ac) of theIEEE 802.11n WLAN system, and is one of IEEE 802.11 WLAN systemsrecently proposed to support a data process speed of 1 Gbps or more at aMAC SAP (Medium Access Control Service Access Point).

In order to efficiently utilize a radio frequency (RF) channel, the nextgeneration WLAN system supports MU-MIMO (Multi User Multiple InputMultiple Output) transmission in which a plurality of STAs cansimultaneously access a channel. In accordance with the MU-MIMOtransmission scheme, the AP may simultaneously transmit packets to atleast one MIMO-paired STA.

In addition, a technology for supporting WLAN system operations inwhitespace has recently been discussed. For example, a technology forintroducing the WLAN system in whitespace (TV WS) such as an idlefrequency band (for example, 54˜698 MHz band) left because of thetransition to digital TV has been discussed under the IEEE 802.11afstandard. However, the above-mentioned information is disclosed forillustrative purposes only, and the whitespace may be a licensed bandcapable of being primarily used only by a licensed user. The licenseduser may be a user who has authority to use the licensed band, and mayalso be referred to as a licensed device, a primary user, an incumbentuser, or the like.

For example, an AP and/or STA operating in the whitespace (WS) mustprovide a function for protecting the licensed user. For example,assuming that the licensed user such as a microphone has already used aspecific WS channel acting as a divided frequency band on regulation ina manner that a specific bandwidth is occupied from the WS band, the APand/or STA cannot use the frequency band corresponding to thecorresponding WS channel so as to protect the licensed user. Inaddition, the AP and/or STA must stop using the corresponding frequencyband under the condition that the licensed user uses a frequency bandused for transmission and/or reception of a current frame.

Therefore, the AP and/or STA must determine whether to use a specificfrequency band of the WS band. In other words, the AP and/or STA mustdetermine the presence or absence of an incumbent user or a licenseduser in the frequency band. The scheme for determining the presence orabsence of the incumbent user in a specific frequency band is referredto as a spectrum sensing scheme. An energy detection scheme, a signaturedetection scheme and the like may be used as the spectrum sensingmechanism. The AP and/or STA may determine that the frequency band isbeing used by an incumbent user if the intensity of a received signalexceeds a predetermined value, or when a DTV preamble is detected.

M2M (Machine to Machine) communication technology has been discussed asnext generation communication technology. Technical standard forsupporting M2M communication has been developed as IEEE 802.11ah in theIEEE 802.11 WLAN system. M2M communication refers to a communicationscheme including one or more machines, or may also be referred to asMachine Type Communication (MTC) or Machine To Machine (M2M)communication. In this case, the machine may be an entity that does notrequire direct handling and intervention of a user. For example, notonly a meter or vending machine including a RF module, but also a userequipment (UE) (such as a smartphone) capable of performingcommunication by automatically accessing the network without userintervention/handling may be an example of such machines. M2Mcommunication may include Device-to-Device (D2D) communication andcommunication between a device and an application server, etc. Asexemplary communication between the device and the application server,communication between a vending machine and an application server,communication between the Point of Sale (POS) device and the applicationserver, and communication between an electric meter, a gas meter or awater meter and the application server. M2M-based communicationapplications may include security, transportation, healthcare, etc. Inthe case of considering the above-mentioned application examples, M2Mcommunication has to support the method for sometimestransmitting/receiving a small amount of data at low speed under anenvironment including a large number of devices.

In more detail, M2M communication must support a large number of STAs.Although the current WLAN system assumes that one AP is associated witha maximum of 2007 STAs, various methods for supporting other cases inwhich many more STAs (e.g., about 6000 STAs) are associated with one APhave recently been discussed in M2M communication. In addition, it isexpected that many applications for supporting/requesting a low transferrate are present in M2M communication. In order to smoothly support manySTAs, the WLAN system may recognize the presence or absence of data tobe transmitted to the STA on the basis of a TIM (Traffic Indicationmap), and various methods for reducing the bitmap size of the TIM haverecently been discussed. In addition, it is expected that much trafficdata having a very long transmission/reception interval is present inM2M communication. For example, in M2M communication, a very smallamount of data (e.g., electric/gas/water metering) needs to betransmitted at long intervals (for example, every month). In addition,the STA operates according to a command received via downlink (i.e., alink from the AP to the non-AP STA) in M2M communication, such that datais reported through uplink (i.e., a link from the non-AP STA to the AP).M2M communication is mainly focused upon the communication schemeimproved on uplink for transmission of the principal data. In addition,an M2M STA is mainly operated as a battery and the user may feeldifficulty in frequently charging the M2M STA with electricity, suchthat battery consumption is minimized, resulting in an increased batterylifetime. In addition, the user may have difficulty in directly handlingthe M2M STA in a specific situation, such that a self-recovery functionis needed. Therefore, although the number of STAs associated with one APincreases in the WLAN system, many developers and companies areconducting intensive research into an WLAN system which can efficientlysupport the case in which there are a very small number of STAs, each ofwhich has a data frame to be received from the AP during one beaconperiod, and at the same time can reduce power consumption of the STA.

As described above, WLAN technology is rapidly developing, and not onlythe above-mentioned exemplary technologies but also other technologiessuch as a direct link setup, improvement of media streaming throughput,high-speed and/or support of large-scale initial session setup, andsupport of extended bandwidth and operation frequency, are beingintensively developed.

WLAN Operating at Sub-1 GHz

As described above, the IEEE 802.11ah standard in which M2Mcommunication is set to a use case has recently been discussed. The IEEE802.11ah standard is operated in an unlicensed band other than a TVwhitespace band at a sub-1 GHz operation frequency, and has a widercoverage (for example, a maximum of 1 km) than a legacy WLAN mainlysupporting a conventional indoor coverage. That is, differently from thelegacy WLAN operated at a frequency of 2.4 GHz or 5 GHz, if a WLAN isoperated at an operation frequency of sub-1 GHz (for example, 700˜900MHz), the AP coverage is increased about two or three times as comparedto the same transmit (Tx) power due to propagation characteristics ofthe corresponding band. In this case, a large number of STAs may beconnected per AP. The Use Case considered in the IEEE 802.11 ah standardcan be summarized as shown in the following Table 1.

TABLE 1 Use Case 1: Sensors and meters 1a: Smart Grid - Meter to Pole1c: Environmental/Agricultural Monitoring 1d: Industrial process sensors1e: Healthcare 1f: Healthcare 1g: Home/Building Automation 1h: Homesensors Use Case 2: Backhaul Sensor and meter data Backhaul aggregationof sensors Backhaul aggregation of industrial sensors Use Case 3:Extended range Wi-Fi Outdoor extended range hotspot Outdoor Wi-Fi forcellular traffic offloading

In accordance with Use Case 1 of Table 1, M2M communication in whichvarious kinds of sensors/meter devices are connected to an 802.11ah APis made available. Specifically, smart grid technology enables a maximumof 6000 sensors/meter devices to be connected to one AP.

In accordance with Use Case 2 of Table 1, an 802.11ah AP configured toprovide a large coverage serves as a backhaul link of a different systemsuch as IEEE 802.15.4g.

In accordance with Use Case 3 of Table 1, Use Case 3 may supportextended home coverage, campus wide coverage, and outdoor extended rangehotspot communication such as shopping-mall range hotspot communication.In accordance with Use Case 3, an 802.11ah AP supports trafficoffloading of cellular mobile communication, such that cellular trafficoverload can be scattered.

A physical layer (PHY) for sub-1 GHz communication is implemented byperforming 1/10 down-clocking of the legacy IEEE 802.11ac PHY. In thiscase, the channel bandwidth of 20/40/80/160/80+80 MHz for use in 802.11ac is provided through 1/10 down-clocking, and the channel bandwidth of2/4/8/16/8+8 MHz is provided at sub-1 GHz. Therefore, a Guard Interval(GI) is increased from 0.8 μs to 8 μs, such that the GI is increased tenfold. The following Table 2 shows the result of comparison between802.11ac PHY throughput and 1/10 down-clocked sub-1 GHz PHY throughput.

TABLE 2 IEEE 802.11ac PHY 1/10 down-clocked sub-1 GHz PHY ChannelBandwidth/Throughput Channel Bandwidth/Throughput 20 MHz/86.7 Mbps 2MHz/8.67 Mbps 40 MHz/200 Mbps  4 MHz/20 Mbps   80 MHz/433.3 Mbps  8MHz/43.33 Mbps 160 MHz/866.7 Mbps 16 MHz/86.67 Mbps 80 + 80 MHz/866.6Mbps    8 + 8 MHz/86.66 Mbps  

Medium Access Mechanism

In the IEEE 802.11-based WLAN system, a basic access mechanism of MAC(Medium Access Control) is a Carrier Sense Multiple Access withCollision Avoidance (CSMA/CA) mechanism. The CSMA/CA mechanism isreferred to as a Distributed Coordination Function (DCF) of IEEE 802.11MAC, and basically includes a “Listen Before Talk” access mechanism. Inaccordance with the above-mentioned access mechanism, the AP and/or STAmay perform Clear Channel Assessment (CCA) for sensing an RF channel ormedium during a predetermined time interval [for example, DCFInter-Frame Space (DIFS)], prior to data transmission. If it isdetermined that the medium is in the idle state, frame transmissionthrough the corresponding medium begins. On the other hand, if it isdetermined that the medium is in the occupied state, the correspondingAP and/or STA does not start its own transmission, establishes a delaytime (for example, a random backoff period) for medium access, andattempts to start frame transmission after waiting for a predeterminedtime. Through application of a random backoff period, it is expectedthat multiple STAs will attempt to start frame transmission afterwaiting for different times, resulting in minimum collision.

In addition, IEEE 802.11 MAC protocol provides a Hybrid CoordinationFunction (HCF). HCF is based on DCF and Point Coordination Function(PCF). PCF refers to the polling-based synchronous access scheme inwhich periodic polling is executed in a manner that all reception (Rx)APs and/or STAs can receive the data frame. In addition, HCF includesEnhanced Distributed Channel Access (EDCA) and HCF Controlled ChannelAccess (HCCA). EDCA is achieved when the access scheme provided from aprovider to a plurality of users is contention-based. HCCA is achievedby the contention-free-based channel access scheme based on the pollingmechanism. In addition, HCF includes a medium access mechanism forimproving Quality of Service (QoS) of WLAN, and may transmit QoS data inboth a Contention Period (CP) and a Contention Free Period (CFP).

FIG. 6 is a conceptual diagram illustrating a backoff process.

Operations based on a random backoff period will hereinafter bedescribed with reference to FIG. 6. If the occupy- or busy-state mediumis shifted to an idle state, several STAs may attempt to transmit data(or frame). As a method for implementing a minimum number of collisions,each STA selects a random backoff count, waits for a slot timecorresponding to the selected backoff count, and then attempts to startdata transmission. The random backoff count is a pseudo-random integer,and may be set to one of 0 to CW values. In this case, CW refers to aContention Window parameter value. Although an initial value of the CWparameter is denoted by CWmin. the initial value may be doubled in caseof a transmission failure (for example, in the case in which ACK of thetransmission frame is not received). If the CW parameter value isdenoted by CWmax, CWmax is maintained until data transmission issuccessful, and at the same time it is possible to attempt to start datatransmission. If data transmission was successful, the CW parametervalue is reset to CWmin. Preferably, CW, CWmin, and CWmax are set to2^(n)−1 (where n=−0, 1, 2, . . . ).

If the random backoff process starts operation, the STA continuouslymonitors the medium while counting down the backoff slot in response tothe decided backoff count value. If the medium is monitored as theoccupied state, the countdown stops and waits for a predetermined time.If the medium is in the idle state, the remaining countdown restarts.

As shown in the example of FIG. 6, if a packet to be transmitted to MACof STA3 arrives at the STA3, the STA3 determines whether the medium isin the idle state during the DIFS, and may directly start frametransmission. In the meantime, the remaining STAs monitor whether themedium is in the busy state, and wait for a predetermined time. Duringthe predetermined time, data to be transmitted may occur in each ofSTA1, STA2, and STA5. If the medium is in the idle state, each STA waitsfor the DIFS time and then performs countdown of the backoff slot inresponse to a random backoff count value selected by each STA. Theexample of FIG. 6 shows that STA2 selects the lowest backoff count valueand STA1 selects the highest backoff count value. That is, after STA2finishes backoff counting, the residual backoff time of STA5 at a frametransmission start time is shorter than the residual backoff time ofSTA1. Each of STA1 and STA5 temporarily stops countdown while STA2occupies the medium, and waits for a predetermined time. If occupying ofthe STA2 is finished and the medium re-enters the idle state, each ofSTA1 and STA5 waits for a predetermined time DIFS, and restarts backoffcounting. That is, after the remaining backoff slot as long as theresidual backoff time is counted down, frame transmission may startoperation. Since the residual backoff time of STA5 is shorter than thatof STA1, STA5 starts frame transmission. Meanwhile, data to betransmitted may occur in STA4 while STA2 occupies the medium. In thiscase, if the medium is in the idle state, STA4 waits for the DIFS time,performs countdown in response to the random backoff count valueselected by the STA4, and then starts frame transmission. FIG. 6exemplarily shows the case in which the residual backoff time of STA5 isidentical to the random backoff count value of STA4 by chance. In thiscase, an unexpected collision may occur between STA4 and STA5. If thecollision occurs between STA4 and STA5, each of STA4 and STA5 does notreceive ACK, resulting in the occurrence of a failure in datatransmission. In this case, each of STA4 and STA5 increases the CW valuetwo times, and STA4 or STA5 may select a random backoff count value andthen perform countdown. Meanwhile, STA1 waits for a predetermined timewhile the medium is in the occupied state due to transmission of STA4and STA5. In this case, if the medium is in the idle state, STA1 waitsfor the DIFS time, and then starts frame transmission after lapse of theresidual backoff time.

STA Sensing Operation

As described above, the CSMA/CA mechanism includes not only a physicalcarrier sensing mechanism in which the AP and/or STA can directly sensethe medium, but also a virtual carrier sensing mechanism. The virtualcarrier sensing mechanism can solve some problems (such as a hidden nodeproblem) encountered in the medium access. For the virtual carriersensing. MAC of the WLAN system can utilize a Network Allocation Vector(NAV). In more detail, by means of the NAV value, the AP and/or STA,each of which currently uses the medium or has authority to use themedium, may inform another AP and/or another STA for the remaining timein which the medium is available. Accordingly, the NAV value maycorrespond to a reserved time in which the medium will be used by the APand/or STA configured to transmit the corresponding frame. An STA havingreceived the NAV value may prohibit or defer medium access (or channelaccess) during the corresponding reserved time. For example, NAV may beset according to the value of a ‘duration’ field of the MAC header ofthe frame.

The robust collision detect mechanism has been proposed to reduce theprobability of such collision, and as such a detailed descriptionthereof will hereinafter be described with reference to FIGS. 7 and 8.Although an actual carrier sensing range is different from atransmission range, it is assumed that the actual carrier sensing rangeis identical to the transmission range for convenience of descriptionand better understanding of the present invention.

FIG. 7 is a conceptual diagram illustrating a hidden node and an exposednode.

FIG. 7(a) exemplarily shows the hidden node. In FIG. 7(a), STA Acommunicates with STA B, and STA C has information to be transmitted. InFIG. 7(a), STA C may determine that the medium is in the idle state whenperforming carrier sensing before transmitting data to STA B, under thecondition that STA A transmits information to STA B. Since transmissionof STA A (i.e., occupied medium) may not be detected at the location ofSTA C, it is determined that the medium is in the idle state. In thiscase, STA B simultaneously receives information of STA A and informationof STA C, resulting in the occurrence of collision. Here, STA A may beconsidered as a hidden node of STA C.

FIG. 7(b) exemplarily shows an exposed node. In FIG. 7(b), under thecondition that STA B transmits data to STA A. STA C has information tobe transmitted to STA D. If STA C performs carrier sensing, it isdetermined that the medium is occupied due to transmission of STA B.Therefore, although STA C has information to be transmitted to STA D,the medium-occupied state is sensed, such that the STA C must wait for apredetermined time (i.e., standby mode) until the medium is in the idlestate. However, since STA A is actually located out of the transmissionrange of STA C, transmission from STA C may not collide withtransmission from STA B from the viewpoint of STA A, such that STA Cunnecessarily enters the standby mode until STA B stops transmission.Here, STA C is referred to as an exposed node of STA B.

FIG. 8 is a conceptual diagram illustrating RTS (Request To Send) andCTS (Clear To Send).

In order to efficiently utilize the collision avoidance mechanism underthe above-mentioned situation of FIG. 7, it is possible to use a shortsignaling packet such as RTS (request to send) and CTS (clear to send).RTS/CTS between two STAs may be overheared by peripheral STA(s), suchthat the peripheral STA(s) may consider whether information iscommunicated between the two STAs. For example, if STA to be used fordata transmission transmits the RTS frame to the STA having receiveddata, the STA having received data transmits the CTS frame to peripheralSTAs, and may inform the peripheral STAs that the STA is going toreceive data.

FIG. 8(a) exemplarily shows the method for solving problems of thehidden node. In FIG. 8(a), it is assumed that each of STA A and STA C isready to transmit data to STA B. If STA A transmits RTS to STA B. STA Btransmits CTS to each of STA A and STA C located in the vicinity of theSTA B. As a result, STA C must wait for a predetermined time until STA Aand STA B stop data transmission, such that collision is prevented fromoccurring.

FIG. 8(b) exemplarily shows the method for solving problems of theexposed node. STA C performs overhearing of RTS/CTS transmission betweenSTA A and STA B, such that STA C may determine no collision although ittransmits data to another STA (for example, STA D). That is, STA Btransmits an RTS to all peripheral STAs, and only STA A having data tobe actually transmitted can transmit a CTS. STA C receives only the RTSand does not receive the CTS of STA A, such that it can be recognizedthat STA A is located outside of the carrier sensing range of STA C (

“STC C”

STA C

).

Power Management

As described above, the WLAN system has to perform channel sensingbefore STA performs data transmission/reception. The operation of alwayssensing the channel causes persistent power consumption of the STA.There is not much difference in power consumption between the reception(Rx) state and the transmission (Tx) state. Continuous maintenance ofthe Rx state may cause large load to a power-limited STA (i.e., STAoperated by a battery). Therefore, if STA maintains the Rx standby modeso as to persistently sense the channel, power is inefficiently consumedwithout special advantages in terms of WLAN throughput. In order tosolve the above-mentioned problem, the WLAN system supports a powermanagement (PM) mode of the STA.

The PM mode of the STA is classified into an active mode and a PowerSave (PS) mode. The STA is basically operated in the active mode. TheSTA operating in the active mode maintains an awake state. If the STA isin the awake state, the STA may normally operate such that it canperform frame transmission/reception, channel scanning, or the like. Onthe other hand, STA operating in the PS mode is configured to switchfrom the doze state to the awake state or vice versa. STA operating inthe sleep state is operated with minimum power, and the STA does notperform frame transmission/reception and channel scanning.

The amount of power consumption is reduced in proportion to a specifictime in which the STA stays in the sleep state, such that the STAoperation time is increased in response to the reduced powerconsumption. However, it is impossible to transmit or receive the framein the sleep state, such that the STA cannot mandatorily operate for along period of time. If there is a frame to be transmitted to the AP,the STA operating in the sleep state is switched to the awake state,such that it can transmit/receive the frame in the awake state. On theother hand, if the AP has a frame to be transmitted to the STA, thesleep-state STA is unable to receive the frame and cannot recognize thepresence of a frame to be received. Accordingly, STA may need to switchto the awake state according to a specific period in order to recognizethe presence or absence of a frame to be transmitted to the STA (or inorder to receive a signal indicating the presence of the frame on theassumption that the presence of the frame to be transmitted to the STAis decided).

FIG. 9 is a conceptual diagram illustrating a power management (PM)operation.

Referring to FIG. 9, AP 210 transmits a beacon frame to STAs present inthe BSS at intervals of a predetermined time period in steps (S211,S212, S213, S214. S215, S216). The beacon frame includes a TIMinformation element. The TIM information element includes bufferedtraffic regarding STAs associated with the AP 210, and includes specificinformation indicating that a frame is to be transmitted. The TIMinformation element includes a TIM for indicating a unicast frame and aDelivery Traffic Indication Map (DTIM) for indicating a multicast orbroadcast frame.

AP 210 may transmit a DTIM once whenever the beacon frame is transmittedthree times. Each of STA1 220 and STA2 222 is operated in the PS mode.Each of STA1 220 and STA2 222 is switched from the sleep state to theawake state every wakeup interval, such that STA1 220 and STA2 222 maybe configured to receive the TIM information element transmitted by theAP 210. Each STA may calculate a switching start time at which each STAmay start switching to the awake state on the basis of its own localclock. In FIG. 9, it is assumed that a clock of the STA is identical toa clock of the AP.

For example, the predetermined wakeup interval may be configured in sucha manner that STA1 220 can switch to the awake state to receive the TIMelement every beacon interval. Accordingly, STA1 220 may switch to theawake state in step S221 when AP 210 first transmits the beacon frame instep S211. STA1 220 receives the beacon frame, and obtains the TIMinformation element. If the obtained TIM element indicates the presenceof a frame to be transmitted to STA1 220. STA1 220 may transmit a PowerSave-Poll (PS-Poll) frame, which requests the AP 210 to transmit theframe, to the AP 210 in step S221 a. The AP 210 may transmit the frameto STA1 220 in response to the PS-Poll frame in step S231. STA1 220having received the frame is re-switched to the sleep state, andoperates in the sleep state.

When AP 210 secondly transmits the beacon frame, a busy medium state inwhich the medium is accessed by another device is obtained, the AP 210may not transmit the beacon frame at an accurate beacon interval and maytransmit the beacon frame at a delayed time in step S212. In this case,although STA1 220 is switched to the awake state in response to thebeacon interval, it does not receive the delay-transmitted beacon frameso that it re-enters the sleep state in step S222.

When AP 210 thirdly transmits the beacon frame, the corresponding beaconframe may include a TIM element denoted by DTIM. However, since the busymedium state is given, AP 210 transmits the beacon frame at a delayedtime in step S213. STA1 220 is switched to the awake state in responseto the beacon interval, and may obtain a DTIM through the beacon frametransmitted by the AP 210. It is assumed that DTIM obtained by STA1 220does not have a frame to be transmitted to STA1 220 and there is a framefor another STA. In this case, STA1 220 confirms the absence of a frameto be received in the STA1 220, and re-enters the sleep state, such thatthe STA1 220 may operate in the sleep state. After the AP 210 transmitsthe beacon frame, the AP 210 transmits the frame to the correspondingSTA in step S232.

AP 210 fourthly transmits the beacon frame in step S214. However, it isimpossible for STA1 220 to obtain information regarding the presence ofbuffered traffic associated with the STA1 220 through double receptionof a TIM element, such that the STA1 220 may adjust the wakeup intervalfor receiving the TIM element. Alternatively, provided that signalinginformation for coordination of the wakeup interval value of STA1 220 iscontained in the beacon frame transmitted by AP 210, the wakeup intervalvalue of the STA1 220 may be adjusted. In this example, STA1 220, thathas been switched to receive a TIM element every beacon interval, may beswitched to another operation state in which STA1 220 can awake from thesleep state once every three beacon intervals. Therefore, when AP 210transmits a fourth beacon frame in step S214 and transmits a fifthbeacon frame in step S215, STA1 220 maintains the sleep state such thatit cannot obtain the corresponding TIM element.

When AP 210 sixthly transmits the beacon frame in step S216, STA1 220 isswitched to the awake state and operates in the awake state, such thatthe STA1 220 is unable to obtain the TIM element contained in the beaconframe in step S224. The TIM element is a DTIM indicating the presence ofa broadcast frame, such that STA1 220 does not transmit the PS-Pollframe to the AP 210 and may receive a broadcast frame transmitted by theAP 210 in step S234. In the meantime, the wakeup interval of STA2 230may be longer than a wakeup interval of STA1 220. Accordingly, STA2 230enters the awake state at a specific time S215 where the AP 210 fifthlytransmits the beacon frame, such that the STA2 230 may receive the TIMelement in step S241. STA2 230 recognizes the presence of a frame to betransmitted to the STA2 230 through the TIM element, and transmits thePS-Poll frame to the AP 210 so as to request frame transmission in stepS241 a. AP 210 may transmit the frame to STA2 230 in response to thePS-Poll frame in step S233.

In order to operate/manage the power save (PS) mode shown in FIG. 9, theTIM element may include either a TIM indicating the presence or absenceof a frame to be transmitted to the STA, or a DTIM indicating thepresence or absence of a broadcast/multicast frame. DTIM may beimplemented through field setting of the TIM element.

FIGS. 10 to 12 are conceptual diagrams illustrating detailed operationsof the STA having received a Traffic Indication Map (TIM).

Referring to FIG. 10, STA is switched from the sleep state to the awakestate so as to receive the beacon frame including a TIM from the AP. STAinterprets the received TIM element such that it can recognize thepresence or absence of buffered traffic to be transmitted to the STA.After STA contends with other STAs to access the medium for PS-Pollframe transmission, the STA may transmit the PS-Poll frame forrequesting data frame transmission to the AP. The AP having received thePS-Poll frame transmitted by the STA may transmit the frame to the STA.STA may receive a data frame and then transmit an ACK frame to the AP inresponse to the received data frame. Thereafter, the STA may re-enterthe sleep state.

As can be seen from FIG. 10, the AP may operate according to theimmediate response scheme, such that the AP receives the PS-Poll framefrom the STA and transmits the data frame after lapse of a predeterminedtime [for example, Short Inter-Frame Space (SIFS)]. In contrast, the APhaving received the PS-Poll frame does not prepare a data frame to betransmitted to the STA during the SIFS time, such that the AP mayoperate according to the deferred response scheme, and as such adetailed description thereof will hereinafter be described withreference to FIG. 11.

The STA operations of FIG. 11 in which the STA is switched from thesleep state to the awake state, receives a TIM from the AP, andtransmits the PS-Poll frame to the AP through contention are identicalto those of FIG. 10. If the AP having received the PS-Poll frame doesnot prepare a data frame during the SIFS time, the AP may transmit theACK frame to the STA instead of transmitting the data frame. If the dataframe is prepared after transmission of the ACK frame, the AP maytransmit the data frame to the STA after completion of such contending.STA may transmit the ACK frame indicating successful reception of a dataframe to the AP, and may be shifted to the sleep state.

FIG. 12 shows the exemplary case in which AP transmits DTIM. STAs may beswitched from the sleep state to the awake state so as to receive thebeacon frame including a DTIM element from the AP. STAs may recognizethat multicast/broadcast frame(s) will be transmitted through thereceived DTIM. After transmission of the beacon frame including theDTIM. AP may directly transmit data (i.e., multicast/broadcast frame)without transmitting/receiving the PS-Poll frame. While STAscontinuously maintains the awake state after reception of the beaconframe including the DTIM, the STAs may receive data, and then switch tothe sleep state after completion of data reception.

TIM Structure

In the operation and management method of the Power save (PS) mode basedon the TIM (or DTIM) protocol shown in FIGS. 9 to 12, STAs may determinethe presence or absence of a data frame to be transmitted for the STAsthrough STA identification information contained in the TIM element. STAidentification information may be specific information associated withan Association Identifier (AID) to be allocated when an STA isassociated with an AP.

AID is used as a unique ID of each STA within one BSS. For example, AIDfor use in the current WLAN system may be allocated to one of 1 to 2007.In the case of the current WLAN system, 14 bits for AID may be allocatedto a frame transmitted by AP and/or STA. Although the AID value may beassigned a maximum of 16383, the values of 2008—16383 are set toreserved values.

The TIM element according to legacy definition is inappropriate forapplication of M2M application through which many STAs (for example, atleast 2007 STAs) are associated with one AP. If the conventional TIMstructure is extended without any change, the TIM bitmap sizeexcessively increases, such that it is impossible to support theextended TIM structure using the legacy frame format, and the extendedTIM structure is inappropriate for M2M communication in whichapplication of a low transfer rate is considered. In addition, it isexpected that there are a very small number of STAs each having an Rxdata frame during one beacon period. Therefore, according to exemplaryapplication of the above-mentioned M2M communication, it is expectedthat the TIM bitmap size is increased and most bits are set to zero (0),such that there is needed a technology capable of efficientlycompressing such bitmap.

In the legacy bitmap compression technology, successive values (each ofwhich is set to zero) of 0 are omitted from a head part of bitmap, andthe omitted result may be defined as an offset (or start point) value.However, although STAs each including the buffered frame is small innumber, if there is a high difference between AID values of respectiveSTAs, compression efficiency is not high. For example, assuming that theframe to be transmitted to only a first STA having an AID of 10 and asecond STA having an AID of 2000 is buffered, the length of a compressedbitmap is set to 1990, the remaining parts other than both edge partsare assigned zero (0). If STAs associated with one AP is small innumber, inefficiency of bitmap compression does not cause seriousproblems. However, if the number of STAs associated with one APincreases, such inefficiency may deteriorate overall system throughput.

In order to solve the above-mentioned problems, AIDs are divided into aplurality of groups such that data can be more efficiently transmittedusing the AIDs. A designated group ID (GID) is allocated to each group.AIDs allocated on the basis of such group will hereinafter be describedwith reference to FIG. 13.

FIG. 13(a) is a conceptual diagram illustrating a group-based AID. InFIG. 13(a), some bits located at the front part of the AID bitmap may beused to indicate a group ID (GID). For example, it is possible todesignate four GIDs using the first two bits of an AID bitmap. If atotal length of the AID bitmap is denoted by N bits, the first two bits(B1 and B2) may represent a GID of the corresponding AID.

FIG. 13(b) is a conceptual diagram illustrating a group-based AID. InFIG. 13(b), a GID may be allocated according to the position of AID. Inthis case, AIDs having the same GID may be represented by offset andlength values. For example, if GID 1 is denoted by Offset A and LengthB, this means that AIDs (A˜A+B−1) on bitmap are respectively set to GID1. For example, FIG. 13(b) assumes that AIDs (1˜N4) are divided intofour groups. In this case, AIDs contained in GID 1 are denoted by 1˜N1,and the AIDs contained in this group may be represented by Offset 1 andLength N1. AIDs contained in GID 2 may be represented by Offset (N1+1)and Length (N2−N1+1), AIDs contained in GID 3 may be represented byOffset (N2+1) and Length (N3−N2+1), and AIDs contained in GID 4 may berepresented by Offset (N3+1) and Length (N4−N3+1).

In case of using the aforementioned group-based AIDs, channel access isallowed in a different time interval according to individual GIDs, theproblem caused by the insufficient number of TIM elements compared witha large number of STAs can be solved and at the same time data can beefficiently transmitted/received. For example, during a specific timeinterval, channel access is allowed only for STA(s) corresponding to aspecific group, and channel access to the remaining STA(s) may berestricted. A predetermined time interval in which access to onlyspecific STA(s) is allowed may also be referred to as a RestrictedAccess Window (RAW).

Channel access based on GID will hereinafter be described with referenceto FIG. 13(c). If AIDs are divided into three groups, the channel accessmechanism according to the beacon interval is exemplarily shown in FIG.13(c). A first beacon interval (or a first RAW) is a specific intervalin which channel access to an STA corresponding to an AID contained inGID 1 is allowed, and channel access of STAs contained in other GIDs isdisallowed. For implementation of the above-mentioned structure, a TIMelement used only for AIDs corresponding to GID 1 is contained in afirst beacon frame. A TIM element used only for AIDs corresponding toGID 2 is contained in a second beacon frame. Accordingly, only channelaccess to an STA corresponding to the AID contained in GID 2 is allowedduring a second beacon interval (or a second RAW) during a second beaconinterval (or a second RAW). A TIM element used only for AIDs having GID3 is contained in a third beacon frame, such that channel access to anSTA corresponding to the AID contained in GID 3 is allowed using a thirdbeacon interval (or a third RAW). A TIM element used only for AIDs eachhaving GID 1 is contained in a fourth beacon frame, such that channelaccess to an STA corresponding to the AID contained in GID 1 is allowedusing a fourth beacon interval (or a fourth RAW). Thereafter, onlychannel access to an STA corresponding to a specific group indicated bythe TIM contained in the corresponding beacon frame may be allowed ineach of beacon intervals subsequent to the fifth beacon interval (or ineach of RAWs subsequent to the fifth RAW).

Although FIG. 13(c) exemplarily shows that the order of allowed GIDs isperiodical or cyclical according to the beacon interval, the scope orspirit of the present invention is not limited thereto. That is, onlyAID(s) contained in specific GID(s) may be contained in a TIM element,such that channel access to STA(s) corresponding to the specific AID(s)is allowed during a specific time interval (for example, a specificRAW), and channel access to the remaining STA(s) is disallowed.

The aforementioned group-based AID allocation scheme may also bereferred to as a hierarchical structure of a TIM. That is, a total AIDspace is divided into a plurality of blocks, and channel access toSTA(s) (i.e., STA(s) of a specific group) corresponding to a specificblock having any one of the remaining values other than ‘0’ may beallowed. Therefore, a large-sized TIM is divided into small-sizedblocks/groups. STA can easily maintain TIM information, andblocks/groups may be easily managed according to class, QoS or usage ofthe STA. Although FIG. 13 exemplarily shows a 2-level layer, ahierarchical TIM structure comprised of two or more levels may beconfigured. For example, a total AID space may be divided into aplurality of page groups, each page group may be divided into aplurality of blocks, and each block may be divided into a plurality ofsub-blocks. In this case, according to the extended version of FIG.13(a), first N1 bits of AID bitmap may represent a page ID (i.e., PID),the next N2 bits may represent a block ID, the next N3 bits mayrepresent a sub-block ID, and the remaining bits may represent theposition of STA bits contained in a sub-block.

In the examples of the present invention, various schemes for dividingSTAs (or AIDs allocated to respective STAs) into predeterminedhierarchical group units, and managing the divided result may be appliedto the embodiments, however, the group-based AID allocation scheme isnot limited to the above examples.

PPDU Frame Format

A Physical Layer Convergence Protocol (PLCP) Packet Data Unit (PPDU)frame format may include a Short Training Field (STF), a Long TrainingField (LTF), a signal (SIG) field, and a data field. The most basic (forexample, non-HT) PPDU frame format may be comprised of a Legacy-STF(L-STF) field, a Legacy-LTF (L-LTF) field, an SIG field, and a datafield. In addition, the most basic PPDU frame format may further includeadditional fields (i.e., STF, LTF, and SIG fields) between the SIG fieldand the data field according to the PPDU frame format types (forexample, HT-mixed format PPDU, HT-greenfield format PPDU, a VHT PPDU,and the like).

STF is a signal for signal detection, Automatic Gain Control (AGC),diversity selection, precise time synchronization, etc. LTF is a signalfor channel estimation, frequency error estimation, etc. The sum of STFand LTF may be referred to as a PCLP preamble. The PLCP preamble may bereferred to as a signal for synchronization and channel estimation of anOFDM physical layer.

The SIG field may include a RATE field, a LENGTH field, etc. The RATEfield may include information regarding data modulation and coding rate.The LENGTH field may include information regarding the length of data.Furthermore, the SIG field may include a parity field, a SIG TAIL bit,etc.

The data field may include a service field, a PLCP Service Data Unit(PSDU), and a PPDU TAIL bit. If necessary, the data field may furtherinclude a padding bit. Some bits of the SERVICE field may be used tosynchronize a descrambler of the receiver. PSDU may correspond to a MACPDU defined in the MAC layer, and may include data generated/used in ahigher layer. A PPDU TAIL bit may allow the encoder to return to a stateof zero (0). The padding bit may be used to adjust the length of a datafield according to a predetermined unit.

MAC PDU may be defined according to various MAC frame formats, and thebasic MAC frame is composed of a MAC header, a frame body, and a FrameCheck Sequence. The MAC frame is composed of MAC PDUs, such that it canbe transmitted/received through PSDU of a data part of the PPDU frameformat.

On the other hand, a null-data packet (NDP) frame format may indicate aframe format having no data packet. That is, the NDP frame includes aPLCP header part (i.e., STF, LTF, and SIG fields) of a general PPDUformat, whereas it does not include the remaining parts (i.e., the datafield). The NDP frame may be referred to as a short frame format.

Single User (SU) Frame/Multi User (MU) Frame

The present invention provides a method for constructing the SIG fieldin each of a SU frame and a MU frame using the WLAN system operating ata frequency of 1 GHz or less (for example, 902˜928 MHz). The SU framemay be used in SU-MIMO, and the MU frame may be used in MU-MIMO. In thefollowing description, the term “frame” may be a data frame or an NDPframe.

FIG. 14 exemplarily shows SU/MU frame formats.

Referring to FIG. 14, STF, LTF1, and SIG-A (SIGNAL A) fields maycorrespond to an omni portion because they are transmitted to all STAsin omni directions. If necessary, beamforming or precoding may not beapplied to STF, LTF1, and SIG-A (SIGNAL A) fields in case of datatransmission.

In the meantime, MU-STF, MU-LTF1, . . . , MU-LTF_N_(LTF), and SIG-B(SIGNAL B) fields located after the SIG-A field are user-specificallytransmitted, and beamforming or precoding may be applied to each fieldbefore such transmission. The MU portion may include MU-STF, MU-LTF(s).SIG-B, and data fields as shown in the frame format of FIG. 14.

In the omni portion, each of STF, LTF1, and SIG-A fields may betransmitted as a single stream in association with each subcarrier, asrepresented by the following equation 1:

[x _(k) ]N _(TX)×1=[Q _(k) ]N _(TX)×1d _(k)  [Equation 1]

In Equation 1, k is a subcarrier (or tone) index, x_(k) is a signaltransmitted at a subcarrier k, and N_(TX) is the number of Tx antennas.Q_(k) is a column vector for encoding (e.g., space-mapping) a signaltransmitted on a subcarrier (k), and d_(k) is data being input to theencoder. In Equation 1, a Cyclic Shift Delay (CSD) of a time domain maybe applied to Q_(k). CSD of the time domain denotes a phase rotation ora phase shift on a frequency domain. Therefore, Q_(k) may include aphase shift value on a tone (k) caused by the time domain CSD.

In the case of using the frame format of FIG. 14, STF, LTF1, and SIG-Afields may be received by all STAs. Each STA may decode the SIG-A fieldthrough channel estimation based on STF and LTF1.

The SIG-A field may include ‘Length/Duration’ information, ‘ChannelBandwidth’ information, and ‘Number of Spatial Streams’ information. TheSIG-A field may have the length of two OFDM symbols. One OFDM symboluses a Binary Phase Shift Keying (BPSK) modulation for 48 data tones,such that 24-bits information may be represented on one OFDM symbol.Accordingly, the SIG-A field may include 48-bits information.

The following Table 3 shows exemplary bit allocation of the SIG-A fieldwith respect to the SU case and the MU case.

TABLE 3 SU MU SU/MU Indication 1 1 Length/Duration 9 9 MCS 4 BW 2 2Aggregation 1 STBC 1 1 Coding 2 5 SGI 1 1 GID 6 Nsts 2 8 PAID 9 ACKIndication 2 2 Reserved 3 3 CRC 4 4 Tail 6 6 Total 48 48

In Table 3, the SU/MU indication field may be used to discriminatebetween the SU frame format and the MU frame format.

The Length/Duration field represents OFDM symbols (i.e., duration) ofthe frame or the number of bytes (i.e., length) of the frame. If theaggregation field of the SU field is set to the value of 1, theLength/Duration field is interpreted as the duration field. In contrast,if the aggregation field is set to zero (0), the Length/Duration fieldis interpreted as the length field. The aggregation field is not definedin the MU frame, and the aggregation field is always applied to the MUfield, such that the Length/Duration field is interpreted as theduration field.

The MCS field indicates the modulation and coding scheme for use in PSDUtransmission. In case of the SU frame, the MCS field is transmittedthrough the SIG-A field. If other STAs (each of which may also bereferred to as 3^(rd) party STA indirectly associated withtransmission/reception between two STAs) are configured to receive theSU frame, the duration of the SU frame (i.e., SU-beamformed frame havingan aggregation field of 0) currently received can be calculated on thebasis of both the length value of the Length/Duration field and thevalue of the MCS field. On the other hand, in the MU field, the MCSfield is not contained in the SIG-A field, and is contained in the SIG-Bfield carrying user-specific information, such that an independent MCSmay be applied for each user.

A BW field represents a channel bandwidth of the SU frame or the MUframe. For example, the BW field may be set to a specific valueindicating one of 2 MHz, 4 MHz, 8 MHz, 16 MH, and 8+8 MHz.

The aggregation field indicates whether a PSDU is aggregated in the formof an aggregation MPDU (i.e., A-MPDU). If the aggregation field is setto 1, this means that a PSDU is aggregated in the form of A-MPDU andthen transmitted. If the aggregation field is set to 0, this means thata PSDU is transmitted without aggregation. In the MU frame, PSDUconfigured in the form of A-MPDU is always transmitted, the aggregationfield need not be signaled, such that the PSDU is not contained in theSIG-A field.

A space time block coding (STBC) field indicates whether STBC is appliedto the SU frame or the MU frame.

The coding field indicates the coding scheme for use in the SU frame orthe MU frame. A Binary Convolutional Code (BCC) scheme, a Low DensityParity Check (LDPC) scheme, etc. may be applied to the SU frame.Independent coding schemes of individual users may be applied to the MUframe, such that the coding field composed of 2 bits or greater may bedefined to support the independent coding schemes.

A short guard interval (SGI) field indicates whether a short GI isapplied to PSDU transmission of the SU frame or the MU frame. In case ofthe MU frame, if SGI is applied to the MU frame, this means that the SGIcan be commonly applied to all users contained in the MU-MIMO group.

The GID field represents Multi-User (MU) group information of the MUframe. In case of the SU frame, a user group need not be defined, sothat the GID field is not contained in the SIG-A field.

A field of the number (Nsts) of space-time streams indicates the numberof space streams of the SU frame or the MU frame. In case of the MUframe, the Nsts field represents the number of space streams of each STAcontained in the corresponding MU group, such that 8 bits are requiredfor the Nsts field. In more detail, a maximum of 4 users may becontained in one MU group and a maximum of space streams may betransmitted to each user, such that 8 bits are needed to correctlysupport the above-mentioned structure.

The partial AID (PAID) field may represent an ID of an STA configured toidentify a reception STA for use in the SU frame. The PAID value in anuplink (UL) frame is composed of some parts of Basic Service Set ID(BSSID). In a downlink (DL) frame, the PAID value may be composed of theAID-hashed result of the STA. For example, BSSID may be 48 bits long,AID may be 16 bits long, and PAID may be 9 bits long.

In addition, according to a new definition and usage of PAID to bedescribed later, PAID of the UL frame may be set to the hashed resultantvalue of some parts of a BSSID, and PAID of the DL frame may be set tothe hashed resultant value of some parts of a BSSID.

The ACK indication field of Table 3 indicates the type of an ACK signalto be transmitted after the SU frame or the MU frame. For example, ifthe ACK indication field is set to 00, this means a normal ACK. If theACK indication field is set to 01, this means a block ACK. If the ACKindication field is set to 10, this means No ACK. However, the ACKindication field is not limited to three ACK types, and may also beclassified into three or more ACK types according to response frameattributes.

In addition, although not shown in Table 3, the SIG field may include aDL/UL indication field (e.g., 1-bit size) explicitly indicating whetherthe corresponding frame is a DL frame or a DL frame. The DL/ULindication field is defined in the SU frame. The DL/UL indication fieldis not defined in the MU frame, and is always used as a DL frame in theMU frame. Alternatively, the SIG field may further include the DL/ULindication field irrespective of types of the SU and MU frames.

Meanwhile, the SIG-B field in the MU frame shown in FIG. 14 may furtherinclude user-specific information. The following Table 4 exemplarilyshows fields used as constituent elements of the SIG-B field of the MUframe. In addition, Table 1 exemplarily shows various parameters appliedto PPDUs of respective bandwidths (BWs) 2, 4, 8 and 16 MHz.

TABLE 4 BW 2 MHz 4 MHz 8 MHz 16 MHz MCS 4 4 4 4 Tail 6 6 6 6 CRC 8 8 8 8Reserved 8 9 11 11 Total 26 27 29 29

In Table 4, an MCS field may indicate an MCS field of a PPDU transmittedin the form of an MU frame per user.

A TAIL bit may enable an encoder to return to a zero (0) state.

The CRC (Cyclic Redundancy Check) field may be used to detect an errorfrom an STA configured to receive the MU frame.

Another Embodiment of SIG Field Bit Allocation

The SIG field applied to the SU/MU frame according to another embodimentof the present invention will be given below.

Table 5 shows another embodiment of the SIG-A field.

TABLE 5 SU MU Duration 9 9 BW 2 2 Aggregation 1 STBC 1 1 Coding 2 5 SGI1 1 GID 6 6 Nsts 2 8 PAID 9 ACK Indication 2 2 Reserved 3 4 CRC 4 4 Tail6 6 Total 48 48

Compared to the SIG-A field of Table 3, the SU/MU indication bit is notshown in Table 5. Instead of the SU/MU indication bit, the GID field maybe used to discriminate between the SU frame and the MU frame as shownin Table 5.

The GID field is contained in the SU frame and the MU frame. If the GIDvalue is set to 0, this means that the corresponding frame is an SUframe transmitted on uplink (e.g., a link from STA to AP). If the GIDvalue is set to 63, this means that the corresponding frame is an SUframe transmitted on downlink (i.e., from AP to STA). If the GID valueis selected from among 1˜62, this means that the corresponding frame isthe MU frame.

In the example of the SIG-A bit allocation shown in Table 5, theLength/Duration field of Table 3 is limited to the Duration field shownin Table 5. In the example of Table 3, if the aggregation field is setto zero (0), the Length/Duration field has the Length value. However,the SIG-A field may be defined to always have the Duration value asshown in the example of Table 5. Meanwhile, for the case in which theaggregation field value is set to zero (0), the length value may becontained in the SIG-B field but not the SIG-A field.

In case of using the SU frame according to the related art, if theaggregation field of the SIG-A field is set to 0, the Length/Durationfield is used as the length field, third party STAs need to decode thedata part so as to recognize duration information of the correspondingframe (in more detail, duration information is contained in the MACheader of the data part). However, if the duration field is defined tobe contained in the SIG-A field of the frame as shown in the example ofTable 5, third party STAs need not decode the data part of the frame soas to calculate a PPDU transmission time of the frame. Since the datapart of the frame need not be decoded, it is not necessary to recognizethe MCS of the corresponding frame. Therefore, the MCS value may not becontained in the SIG-A corresponding to the omni portion, and may becontained in the SIG-B including user-specific information. Accordingly,the example of Table 5 may not include the MCS field as compared to theexample of Table 3.

The SIG-B field of the MU frame includes user-specific information, andmay be defined in the same manner as in Table 4.

Differently from the example of Table 4, the Length field and the MCSfield are contained in the SIG-B field of the SU frame. The followingTable 6 shows bit allocation of the SIG-B of the SU frame according toanother example of the present invention.

TABLE 6 BW 2 MHz 4 MHz 8 MHz 16 MHz MCS 4 4 4 4 Length 9 9 9 9 Tail 6 66 6 CRC 4 4 4 4 Reserved 3 4 6 6 Total 26 27 29 29

In Table 6, the MCS field represents the modulation and coding schemefor use in PSDU transmission.

The Length field of Table 6 represents the number of bytes of PSDU, andmay be used in case of the aggregation level (i.e., a value of theaggregation field of SIG-A) of 0. However, the scope or spirit of theLength field is not limited thereto. Even if the aggregation field isset to 1, the Length field may be contained in the SIG-B field.

In Table 6, the CRC field of the SIG-B bit allocation may be defined tohave 4 bits long in the same manner as in SIG-A (e.g., Table 5). Whilethe CRC field is 8 bits long in the example of Table 4, it should benoted that the CRC field may be defined to have 4 bits long so as toguarantee reserved bits of the SIG-B as shown in the example of Table 6.

New Definition and Usage of PAID

PAID is a non-unique identifier of an STA. As shown in Table 3 or 5,PAID may be contained in the SU frame. In more detail, the PAID may becontained in the SU frame defined in a sub-1 GHz operation frequency.PAID may be limited to 9 bits long.

The embodiment of the present invention provides a method fordiscriminating between a DL frame and a UL frame using the PAID field.The above-mentioned embodiment can be efficiently applied to the case inwhich the DL/UL indication field is not contained in the SIG field. Theembodiment of the present invention can define a method fordiscriminating between the DL frame and the UL frame using the PAIDalthough the DL/UL indication field is contained in the SIG field, suchthat the above-mentioned embodiment can still be efficiently utilized.That is, the PAID definition and usage method according to the presentinvention may be utilized irrespective of the presence or absence ofDL/UL indicators.

As described above, if a GID value is set to any one of 0˜63, each UL SUframe (i.e., SU frame in which an intended receiver is an AP) and eachDL SU frame (i.e., SU frame in which an intended receiver is an STA) maybe defined. In the meantime, if a GID value is set to any one of 1˜62,this means an MU frame. However, if the GID field is not present and theDL/UL indicator field is not present, this means that it is impossibleto discriminate between DL and UL of the SU frame according to therelated art.

In order to solve the above problems, the present invention provides amethod for determining whether the corresponding frame is a DL frame ora UL frame using a value of the PAID field. Values of the PAID field maybe defined in Table 7 according to examples of the present invention.

TABLE 7 Condition Partial AID Addressed to AP (dec(BSSID[39:47])mod(2{circumflex over ( )}9−1)) + 1 Addressed to Mesh STA(dec(BSSID[39:47])mod (2{circumflex over ( )}9−1)) + 1 Sent by an AP and(dec(AID[0:8] + addressed to a STA dec(BSSID[44:47]XORBSSID[40:43]) ×associated with that AP 2{circumflex over ( )}5)mod 2{circumflex over( )}9 or sent by a DLS or where mod X indicates the X-modulo TDLS STA ina direct operation, dec(A[b:c]) is the cast to path to a DLS or decimaloperator where b is scaled by 2{circumflex over ( )}0 and c TDLS STA by2{circumflex over ( )}(c−b). Otherwise 0

In the example of Table 7, a method for calculating a PAID value of eachframe type is defined.

Referring to Table 7, the PAID value in the case in which the STAtransmits a UL frame to the AP may be calculated as follows.

(1) 9 bits from the 40^(th) bit to the 48^(th) bit are extracted from aBSSID of the AP. In this case, if the bit index starts from Bit 0, theposition of the 40^(th) bit corresponds to Bit 39, and the position ofthe 48^(th) bit corresponds to Bit 47. Alternatively, if the bit indexstarts from Bit 1, the position of the 40^(th) bit corresponds to Bit40, and the position of the 48^(th) bit corresponds to Bit 48. Thefollowing examples assume the bit index starting from Bit 0 forconvenience of description and better understanding of the presentinvention, it should be noted that the principles of the presentinvention may also be applied to the case in which a bit index startsfrom Bit 1.

(2) 9 extracted bits are converted into a decimal number. Conversion toa decimal number may be denoted by dec(A), and dec(A) is a specificvalue obtained when A is converted into a decimal number.

(3) The operation ‘mod (2̂9−1)’ is applied to the converted decimalnumber. Here, “mod” is a modulo operation, “X mod Y” is the remainderwhen X is divided by Y, and 2̂9=2⁹=512, and 2⁹−1=511. Therefore, theresultant value of the Step (3) is set to one of 0 to 510.

(4) The value of 1 is added to the resultant value of the mod (2̂9−1)operation, and the added result is set to one of 1 to 511, such that theresultant value is the final result value acting as a PAID.

The above-mentioned steps may be represented by the following equation2:

dec(BSSID[39:47])mod(2⁹−1)+1  [Equation 2]

The reason why the PAID value is calculated as shown in the Equation 2is to prevent the PAID value from being zero. PAID=0 is used for otherusages such as multicast/broadcast.

In the example of Table 7, a PAID in frame transmission between meshSTAs is calculated as follows. 9 bits from the 40^(th) bit to the48^(th) bit of a BSSID of the counterpart mesh STA acting as a peer areconverted into a decimal number, the value of 1 is added to theresultant value of the mod (2̂9−1) operation, and the added result is setto one of 1 to 511, such that the resultant value is the final resultvalue acting as a PAID. That is, while Equation 2 is equally applied toframe transmission to the mesh STA, a BSSID of the AP may be replacedwith a BSSID of the mesh STA according to the PAID calculation scheme ofthe UL frame.

In a first case in which the AP transmits a DL frame to the STA and in asecond case in which the STA transmits a frame through a Direct LinkSetup (DLS)/Tunneled Direct Link Setup (TDLS) link on a direct path, thePAID is calculated as shown in the following equation 3:

(dec(AID[0:8])+dec(BSSID[44:47]XOR BSSID[40:43])×2⁵)mod 2⁹  [Equation 3]

In Equation 3, XOR is an exclusive OR operation. For example, 1 XOR 1=0,0 XOR 1=1, 1 XOR 0=1, and 0 XOR 0=0 may be calculated by Equation 3.

In case of the DL frame or the DLS/TDLS frame, partial information ofBSSID and AID is hashed as shown in Equation 3, such that the hashedresultant value is used as a PAID. In more detail, 9 bits from the firstbit position to the ninth bit position of the AID are converted into adecimal number (i.e., dec(AID[0:8])). In addition, the resultant value(i.e., BSSID[44:47] XOR BSSID[40:43]) obtained when 4 bits (i.e.,BSSID[44:47]) from the 45^(th) bits to the 48^(th) bit of a BSSID areXOR-operated with 4 bits (i.e., BSSID[40:43]) from the 41^(th) bit tothe 44^(th) bit of a BSSID is converted into a decimal number (i.e.,dec(BSSID[44:47] XOR BSSID[40:43])). In the above-mentioned calculationresult, the XOR result based on BSSID is 4 bits long, and is convertedinto a decimal number. 9 bits in AID are converted into a decimalnumber, 2⁵ is multiplied by the decimal resultant value obtained from aBSSID so as to adjust a digit number (where multiplication of 2⁵ isconceptually identical to the case in which the length of 5 bits isadded to a binary number). Therefore, the result obtained on the basisof a BSSID is added to the result obtained from the AID. The mod 2⁹operation is performed on the added result, so that one of 0˜511 may beset to a PAID.

The term “Otherwise” shown in Table 7 means that a broadcast/multicastframe transmitted to all STAs by an AP or a frame transmitted by anon-associated STA is used. In this case, a PAID value is set to zero(0).

If the PAID value is calculated according to a predetermined conditionas shown in Table 7, an AP considers only a specific frame in which thePAID value is identical to 0 or ‘dec(BSSID[39:47])mod(2⁹−1))+1’ to be aframe having a high possibility that the frame is transmitted to the AP,and then decodes a PSDU.

In addition, an STA considers only a specific frame in which the PAIDvalue is identical to 0 or dec(AID[0:8]+dec(BSSID[44:47] XORBSSID[40:43])×2⁵)mod 2⁹ to be a frame having a high possibility that theframe is transmitted to the STA, and then decodes a PSDU.

In this case, when the AP allocates the AID to the STA, it is preferablethat a specific AID through which the resultant value of Equation 3calculated by an allocated AID is set to zero (0) may not be allocatedto the STA. If the AID through which the calculation result of Equation3 is set to zero (0) is allocated to the STA, a PAID value of the frametransmitted to the corresponding STA is set to 0, such that all otherSTAs consider the corresponding frame to be a multicast/broadcast frameirrespective of a receiver of the corresponding frame, and thus attemptto perform unnecessary PSDU decoding of the corresponding frame.Therefore, an AID through which the resultant value of Equation 3 is setto 0 so as to distinguish a current frame from a different type of frameneed not be allocated to the STA.

In addition, when the AP allocates an AID to the STA, it is preferablethat a specific AID by which a first calculation result (i.e.,dec(BSSID[39:47])mod(2⁹−1))+1) of the Equation 2 based on a BSSID of theAP is identical to a second calculation result (i.e.,dec(AID[0:8]+dec(BSSID[44:47] XOR BSSID[40:43])×2⁵)mod 2⁹) of theEquation 3 based on an AID allocated to the STA and a BSSID of the APmay not be allocated to the STA. In case of allocating an AID of aspecific STA in a manner that a PAID value (i.e., the calculation resultof Equation 2) for the UL frame is identical to a PAID value (i.e., thecalculation result of Equation 3) of the DL frame, the specific STAconsiders UL frames transmitted from other STAs to the AP to be DLframes for the specific STA, such that the STA attempts to performunnecessary PSDU decoding of the corresponding frames.

In addition, when an overlapping BSS is present, an AP (i.e., APassociated with STA) may allocate an AID of the STA in consideration ofan OBSS BSSID of the AP of an OBSS (i.e., a BSS operated at the samechannel as that of a BSS of the AP associated with the STA, and mayoverlap some or all of a BSA) constructing an OBSS. That is, when the APallocates the AID to the STA, a specific AID, through which thecalculation result (i.e., (dec(AID[0:8]+dec(BSSID[44:47] XORBSSID[40:43])×2⁵)mod 2⁹) of Equation 3 on the basis of both an AIDallocated to the STA and a BSSID of the AP is identical to thecalculation result (i.e., dec(OBSS BSSID[39:47])mod (2⁹−1))+1) ofEquation 2 on the basis of a BSSID of an OBSS, is not allocated to theSTA. (Preferably, an AP should not assign an AID to a STA that resultsin the PARTIAL_AID value, as computed using Equation 3, being equal toeither (dec(BSSID[39:47])mod(29−1))+1 or (dec(OverlappingBSSID[39:47])mod(29−1))+1.) Otherwise, an STA considers all OBSS ULframes transmitted from STAs contained in an OBSS to an OBSS AP to be DLframes to be transmitted for the STA, such that it attempts to performunnecessary PSDU decoding of the corresponding frames.

In other words, when the AP allocates an AID to the STA, a PAID value tobe obtained when Equation 3 (i.e., dec(AID[0:8]+dec(BSSID[44:47] XORBSSID[40:43])×2⁵)mod 2⁹) is applied to an AID to be allocated should notbe identical to each of (dec(BSSID[³9:47])^(m)od(2⁹−1))+1 and (dec(OBSSBSSID[39:47])mod(2⁹−1))+1. That is, not only a first AID through which aPAID value obtained when (dec(AID[0:8]+dec(BSSID[44:47] XORB^(S)SID[40:43])×2⁵)mod 2⁹ is applied to an AID value to be allocated isidentical to (dec(BSSID[39:47])mod(2⁹−1))+1, but also a second AIDthrough which the obtained PAID value is identical to (dec(OBSSBSSID[39:47])mod(2⁹−1))+1 is excluded from the AID to be allocated, suchthat an AID selected from among the remaining AIDs must be allocated tothe STA.

In order to prevent the OBSS collision problem from occurring, the APhas to recognize a BSSID (i.e., OBSS BSSID) of the OBSS AP. However, ifthe AP does not detect the OBSS AP, a specific AID through which thecalculation result of Equation 3 on the basis of the AID allocated tothe AP-associated STA is identical to the calculation result of Equation3 on the basis of the OBSS BSSID may also be allocated to the STA. Inthis case, the STA may request the AP to change a current AID to anotherAID.

For example, assuming that a specific AID through which the calculationvalue of (dec(AID[0:8]+dec(BSSID[44:47] XOR BSSID[40:43])×2⁵)mod 2⁹ isidentical to dec(OBSS BSSID[39:47])mod (2⁹−1))+1 is allocated to theSTA, the STA may transmit an AID reassignment request frame to the AP.If the AP receives the AID reassignment request frame and a reason codeof the AID reassignment request frame indicates “Partial AID Collision”,the AP does not allocate the corresponding AID value to STAs. The APtransmits the AID reassignment response frame to the corresponding userequipment (UE), such that it may allocate a new AID to the UE. Adetailed example of the present invention will be given in the following“AID reassignment request/response” item.

In order to enable a PAID to be used for appropriate purposes inconsideration of the above-mentioned cases, when the AP allocates an AIDto UEs, a DL-frame PAID value (i.e., the calculation result of Equation31) obtained through the hashed result of AID and BSSID should notoverlap a PAID value (e.g., zero 0) designated for a specific frame typesuch as a multicast/broadcast type, and should not overlap a PAID value(i.e., the calculation result of Equation 2) of the UL frame transmittedto either an AP (i.e., associated AP) or an OBSS AP. In addition, inorder to prevent collision from being generated, an AID corresponding tothe above three conditions is not allocated as an AID of each STA, andmay be used for separate usages (e.g., multicast frame).

In addition, a PAID value of the AP may be selected as an arbitrary onefrom among values of a specific range. In case of a PAID value of theAP, a DL-frame PAID value obtained through the hashed result of AID andBSSID should not be identical to a PAID value designated for a specificSTA as in the UL frame transmitted to an AP or an OBSS AP.

In accordance with additional embodiments of the example of Table 7, aPAID value to be used for a specific-type frame (for example, a beaconframe, a probe response frame, etc.) may be pre-designated, and a PAIDfor a normal frame may not be set to a predetermined value for thespecific-type frame. Associated examples are shown in Table 8.

TABLE 8 Condition Partial AID Addressed to AP (dec(BSSID[39:47])mod(2{circumflex over ( )}9−1−k)) + 1 + k Addressed to Mesh STA(dec(BSSID[39:47])mod (2{circumflex over ( )}9−1−k)) + 1 + k Sent by anAP and (dec(AID[0:8] + addressed to a STAdec(BSSID[44:47]XORBSSID[40:43]) × associated with that AP 2{circumflexover ( )}5)mod 2{circumflex over ( )}9 or sent by a DLS or where mod Xindicates the X-modulo TDLS STA in a direct operation, dec(A[b:c]) isthe cast to path to a DLS or decimal operator where b is scaled by2{circumflex over ( )}0 and c TDLS STA by 2{circumflex over ( )}(c−b).Special frame 1 . . . k (e.g., Beacon frame or Probe Response frame)Otherwise 0

If PAID #1 is allocated to a beacon frame and PAID #2 is allocated to aprobe response frame as shown in the example of Table 8, the AP mustallocate PAIDs (0, 1, 2) to a DL frame and must allocate an AID not usedin a UL frame to each of STAs.

For this purpose, as shown in the example of Table 8, in order toprevent a PAID of the UL frame (i.e., a frame directed to the AP) frombeing set to the value of 0, 1, or 2, PAID can be calculated as shown inthe following Equation 4.

dec(BSSID[39:47])mod(2⁹−1−2)+1+2  [Equation 4]

Equation 4 may represent the case in which k=2 is given in the exampleof Table 8. That is, 9 bits from the 40^(th) bit to the 48^(th) bit ofan AP BSSID are denoted by a decimal number (i.e., dec([39:47])), andthe mod(2⁹−1−2) operation is applied to the resultant decimal number, sothat the values of 0 to 508 are obtained, 3 is added to the values 0 to508 to result in 3 to 511. As a result, one of 3 to 511 may be used as aPAID value of the UL frame.

In accordance with an additional proposal of the present invention, aspecific part that uses 9 bits from the 40^(th) bit position to the48^(th) bit position (i.e., [39:47]) of an AP BSSID may also be replacedwith a different value obtained when 8 bits from the 41^(st) bitposition to the 48^(th) bit position (i.e., [40:47]) of an AP BSSID isconcatenated with a binary value of 1. In this case, the bit position atwhich concatenation of the binary value 1 occurs may represent a LeastSignificant Bit (LSB) or Most Significant Bit (MSB) corresponding to 8bits.

AID Reassignment Request/Response

FIG. 15(a) exemplarily shows an example of an AID reassignment requestframe format, and FIG. 15(b) exemplarily shows an example of an AIDreassignment response frame format.

Referring to FIG. 15(a), the Category field may be set to a specificvalue indicating a category associated with the corresponding frame. TheAction field may be set to a specific value indicating which one ofmanagement operations contained in the above category field isassociated with the corresponding frame.

The reason code may be set to a specific value indicating Partial AID(PAID) Collision. When an STA transmits the AID reassignment requestframe to the AP, the STA further transmits Partial BSSID informationregarding a BSSID of all OBSS APs detected by the STA, such thatcollision between the STA and each of the OBSS BSSIDs is prevented fromoccurring when the AP reassigns an AID for the corresponding STA.

‘OBSS Partial BSSID List’ field may include 8 bits [40:47] from amongOBSS BSSID.

In response to the AID reassignment request frame, the AP may transmitan AID reassignment response frame having a frame format shown in FIG.15(b) to the STA.

Referring to FIG. 15(b), the Category field may be set to a specificvalue indicating a category associated with the corresponding frame. TheAction field may be set to a specific value indicating which one ofmanagement operations contained in the above category field isassociated with the corresponding frame.

The New AID field may include an AID that is newly allocated from the APto the STA. When the AP allocates a new AID, a new AID must be allocatedin such a manner that OBSS BSSIDs known through the OBSS Partial BSSIDlist of the AID reassignment request frame do not collide with a PAIDvalue calculated on the basis of a newly allocated AID.

The AID Activation Offset field indicates a time offset consumed untilreaching a specific time at which a newly allocated AID value isactually used. Such a time offset unit may be represented in units of abeacon interval, a DTIM beacon interval, or a time unit. TU may beconfigured in units of microsecond (μs), for example, 1024 μs.

The Duty Cycle field represents a duty cycle of an AID and a grouphaving the AID, and may include a sleep interval or an inactivationduration.

Additional Proposal of SIG-B Field of the SU Field

In case of the SU frame, all information for decoding the correspondingframe may be contained in the SIG-A field, such that the SIG-B field maybe no longer required. Accordingly, the SU frame format may beconfigured according to a specific communication scheme in which theSIG-B field is not transmitted in the SU-frame.

However, definition of a separate format other than the SIG-B field mayunavoidably increase the processing load of a frame generation part anda frame reception/decoding part, such that the SU frame is configured toinclude the SIG-B field as in the legacy art, and the constituentcontent of the SU frame may be configured as follows. The SIG-B fieldneed not include substantial control information, such that the sequenceof a fixed pattern for reducing a Peak to Average Power Ratio (PAPR) isdefined and repeated, resulting in construction of the SIG-B field.

FIG. 16 exemplarily shows fixed sequences available as an SIG-B field.

FIG. 16(a) exemplarily shows examples of the fixed sequence available asthe SIG-B field of 2 MHz PPDU. FIG. 16(b) exemplarily shows examples ofthe fixed sequences available as the SIG-B field of 4 MHZ PPDU. FIG.16(c) exemplarily shows examples of the fixed sequences available as theSIG-B field of 8 MHz/16 MH/8+8 MHz PPDU.

Referring to FIG. 16(a), in the case of using a PPDU of a 2 MHzbandwidth, a total bit length of the SIG-B field is denoted by 26 asshown in Table 4, and 6 bits from among 26 bits are allocated to TAILbits. Therefore, the length of a fixed sequence may be 20 bits long(i.e., B0 to B19). The sequence pattern of FIG. 16(a) is only exemplary,and the scope or spirit of the present invention is not limited thereto.

Referring to FIG. 16(b), in the case of using a PPDU of a 4 MHzbandwidth, a total bit length of the SIG-B field is denoted by 27 asshown in Table 4, and 6 bits from among 27 bits are allocated to TAILbits. Therefore, the length of a fixed sequence may be 21 bits long(i.e., B0 to B20). The sequence pattern of FIG. 16(b) is only exemplary,and the scope or spirit of the present invention is not limited thereto.

Referring to FIG. 16(c), in the case of using a PPDU of a 8 MHz/16MH/8+8 MHz bandwidth, a total bit length of the SIG-B field is denotedby 29 as shown in Table 4, and 6 bits from among 29 bits are allocatedto TAIL bits. Therefore, the length of a fixed sequence may be 23 bitslong (i.e., B0 to B22). The sequence pattern of FIG. 16(c) is onlyexemplary, and the scope or spirit of the present invention is notlimited thereto.

FIG. 17 is a conceptual diagram illustrating a method repeated in theSIG-B field when a fixed pattern of FIG. 16 is transmitted to PPDU.

In the example of 2 MHz PPDU of FIG. 17, 20 bits may correspond to afixed sequence pattern of FIG. 16(a).

In the 4 MHz PPDU example of FIG. 17, 21 bits may correspond to a fixedsequence pattern of FIG. 16(b). Here, 4 MHz PPDU exemplarily shows thatthe SIG-B field (i.e., fixed sequence+TAIL) is repeated once more (i.e.,a total of twice transmission).

In the 8 MHz PPDU example of FIG. 17, 23 bits may correspond to thefixed sequence pattern of FIG. 16(c). Here, 8 MHz PPDU exemplarily showsthat the SIG-B field (i.e., fixed sequence+TAIL) is repeated three timesmore (i.e., the SIG-N field is transmitted four times). A padding bit of1 bit may be added to the SIG-B field in a manner that a total lengthobtained by repetition of the SIG-B field is adjusted to a predeterminedlength.

In the 16 MHz PPDU example of FIG. 17, 23 bits may correspond to thefixed sequence pattern of FIG. 16(c). Here, 16 MHz PPDU exemplarilyshows that the SIG-B field (i.e., fixed sequence+TAIL) is repeated threetimes more (i.e., the SIG-B field is transmitted four times) and the setto which the padding is added is repeated once more (i.e., twicetransmission).

FIG. 18 is a flowchart illustrating a method for transmitting andreceiving a frame according to one embodiment of the present invention.

Referring to FIG. 18, an STA may calculate a PAID contained in a frame(i.e., UL frame) addressed to an AP on the basis of a BSSID of an AP asshown in the example of the present invention in step S1810. Forexample, the resultant value of Equation 2 (i.e.,(dec(BSSID[39:47])mod(2⁹−1))+1) may be contained in a PAID field.Accordingly, a PAID of the UL SU frame may be set to a value but not ‘0’(e.g., any one of 1 to 511). In this case, the UL SU frame may bedefined as a frame transmitted at sub-1 GHz operation frequency band.

In step S1820, the STA constructs a PAID field including a PAID valuecalculated at step S1810 and an SIG-A field including various fieldsproposed by Table 3, and may transmit a frame constructed according tothe SU frame format (e.g., SU PPDU frame format) including other fieldsto the AP.

In step S1830, the AP may receive the frame, and may confirm a PAIDfield of the SIG-A field of the frame. That is, the AP may determinewhether the PAID value is calculated on the basis of a BSSID of the AP.

In step S1840, if it is determined that the PAID field value of theframe of step S1830 is calculated (e.g., calculated according toEquation 2) on the basis of a BSSID of the AP, the AP may perform PSDUdecoding of the PPDU frame.

In the meantime, the AP may calculate a PAID value to be contained in aframe to be transmitted to the STA in step S1850. The PAID valuecontained in the frame to be transmitted by the AP may be calculated onthe basis of both an AID allocated to the STA and a BSSID of the AP(e.g., dec(AID[0:8]+dec(BSSID[44:47] XOR BSSID[40:43])×2⁵)mod 2⁹).

In this case, the AID value allocated to the STA must be allocated in amanner that the PAID value calculated using the corresponding AID valueis not identical to each of a first PAID and a second PAID.

The first PAID may be a specific value (i.e.,(dec(BSSID[39:47])mod(2⁹−1))+1) calculated by applying the modulooperation to the resultant value obtained when values from the 40^(th)bit position and the 48^(th) bit position from among 48 bit positions ofa BSSID of the AP are converted into a decimal number. The second PAIDmay be a specific value (i.e., (dec(OBSS BSSID[39:47])mod(2⁹−1))+1)calculated by applying the modulo operation to the resultant valueobtained when values from the 40^(th) bit position to the 48^(th) bitposition from among 48 bit positions of a BSSID of the OBSS areconverted into a decimal number.

In step S1860, the AP constructs a PAID field including a PAID valuecalculated at step S1850 and an SIG-A field including various fieldsproposed by Table 3, and may transmit a frame constructed according tothe SU frame format (e.g., SU PPDU frame format) including other fieldsto the STA.

In step S1870, the STA may receive the frame, and may confirm a PAIDfield of the SIG-A field of the frame. That is, the STA may determinewhether a PAID value is calculated on the basis of an AID allocated tothe STA by the AP and a BSSID of the AP.

In step S1880, if it is determined that the PAID field value of theframe of step S1870 is calculated (e.g., calculated according toEquation 3) on the basis of an AID of the STA and a BSSID of the AP, theSTA may perform PSDU decoding of the PPDU frame.

In a method (specifically, the PAID construction method) fortransmitting and receiving a frame according to the present invention asshown in FIG. 18, various embodiments of the present invention areperformed independently or two or more embodiments of the presentinvention are performed simultaneously.

FIG. 19 is a block diagram illustrating a radio frequency (RF) deviceaccording to one embodiment of the present invention.

Referring to FIG. 19, an AP 10 may include a processor 11, a memory 12,and a transceiver 13. An STA 20 may include a processor 21, a memory 22,and a transceiver 23. The transceivers 13 and 23 may transmit/receiveradio frequency (RF) signals and may implement a physical layeraccording to an IEEE 802 system. The processors 11 and 21 are connectedto the transceivers 13 and 21, respectively, and may implement aphysical layer and/or a MAC layer according to the IEEE 802 system. Theprocessors 11 and 21 may be configured to operate according to theabove-described various embodiments of the present invention. Modulesfor implementing operation of the AP and STA according to theabove-described various embodiments of the present invention are storedin the memories 12 and 22 and may be implemented by the processors 11and 21. The memories 12 and 22 may be included in the processors 11 and21 or may be installed at the exterior of the processors 11 and 21 to beconnected by a known means to the processors 11 and 21.

The overall configuration of the AP and STA may be implemented such thatabove-described various embodiments of the present invention may beindependently applied or two or more embodiments thereof may besimultaneously applied and a repeated description is omitted forclarity.

The above-described embodiments may be implemented by various means, forexample, by hardware, firmware, software, or a combination thereof.

In a hardware configuration, the method according to the embodiments ofthe present invention may be implemented by one or more ApplicationSpecific Integrated Circuits (ASICs), Digital Signal Processors (DSPs),Digital Signal Processing Devices (DSPDs), Programmable Logic Devices(PLDs), Field Programmable Gate Arrays (FPGAs), processors, controllers,microcontrollers, or microprocessors.

In a firmware or software configuration, the method according to theembodiments of the present invention may be implemented in the form ofmodules, procedures, functions, etc. performing the above-describedfunctions or operations. Software code may be stored in a memory unitand executed by a processor. The memory unit may be located at theinterior or exterior of the processor and may transmit and receive datato and from the processor via various known means.

The detailed description of the preferred embodiments of the presentinvention has been given to enable those skilled in the art to implementand practice the invention. Although the invention has been describedwith reference to the preferred embodiments, those skilled in the artwill appreciate that various modifications and variations can be made inthe present invention without departing from the spirit or scope of theinvention described in the appended claims. Accordingly, the inventionshould not be limited to the specific embodiments described herein, butshould be accorded the broadest scope consistent with the principles andnovel features disclosed herein.

INDUSTRIAL APPLICABILITY

Although the above various embodiments of the present invention havebeen described based on an IEEE 802.11 system, the embodiments may beapplied in the same manner to various mobile communication systems.

1.-14. (canceled)
 15. A method of transmitting a frame to a station(STA) in a wireless communication system, the method performed by anaccess point (AP) and comprising: allocating, by the AP, an associationidentifier (AID) to the STA; and transmitting, by the AP, a downlinkframe including a partial AID (PAID) of the STA which is obtained byusing the allocated AID, wherein in the allocation of the AID, the APexcludes a predetermined AID value which makes the PAID to be‘BSSID[39:47] mod(2⁹−1)+1’, where ‘BSSID’ denotes either an ID of abasic service set (BSS) to which AP belongs or an ID of an overlappingBSS (OBSS).
 16. The method of claim 15, wherein the PAID of the STA is‘(AID[0:8]+2⁵×BSSID[44:47] XOR BSSID[40:43])mod 2⁹’.
 17. The method ofclaim 15, wherein the AP allocates the AID such that the PAID of the STAis not identical to zero.
 18. The method of claim 15, wherein thedownlink frame corresponds to a single user physical layer protocol dataunit (SU PPDU).
 19. The method of claim 15, wherein the PAID is includedin a signal (SIG) field of the downlink frame.
 20. The method of claim15, further comprising: receiving a uplink frame including a PAIDcorresponding to ‘BSSID[39:47] mod(2⁹−1)+1’.
 21. An access point (AP)comprising: a processor to allocate an association identifier (AID) to astation (STA); and a transmitter to transmit a downlink frame includinga partial AID (PAID) of the STA which is obtained by using the allocatedAID, wherein in the allocation of the AID, the processor excludes apredetermined AID value which makes the PAID to be ‘BSSID[39:47]mod(2⁹−1)+1’, where ‘BSSID’ denotes either an ID of a basic service set(BSS) to which AP belongs or an ID of an overlapping BSS (OBSS), and‘dec( )’ denotes a decimal conversion.
 22. The AP of claim 21, whereinthe PAID of the STA is ‘(AID[0:8]+2⁵×BSSID[44:47] XOR BSSID[40:43])mod2⁹’.
 23. The AP of claim 21, wherein the AP allocates the AID such thatthe PAID of the STA is not identical to zero.
 24. The AP of claim 21,wherein the downlink frame corresponds to a single user physical layerprotocol data unit (SU PPDU).
 25. The AP of claim 21, wherein the PAIDis included in a signal (SIG) field of the downlink frame.
 26. The AP ofclaim 21, further comprising: a receiver to receive a uplink frameincluding a PAID corresponding to ‘BSSID[39:47] mod(2⁹−1)+1’.